And now here's the other side of the argument...
I fully agree that there are circumstances, prolly many circumstances, where mental speed is not a limiting factor for feruchemical steel. Running along a typical path at the speed of a sprinter for miles and miles. Making a sandwich. Trying every key from a ring in a lock. But there are other circumstances where mental speed DOES become a strong limiting factor, relatively quickly. Writing a note, you can't write any faster than your mind can think of the words. Picking a lock, when you have to notice and react to subtle changes in the lock itself. And, I think, combat, when you're matching wits with someone as much as you're trading blows.
There is an inherent flaw in Kurk's "Flash-stepping" example. It's not an example of super-human speed matched with no-more-than-human reflexes, it's super-human speed matched with super-human reflexes. You can be facing a monster, being reasonably sure that his back is a weak spot, and run around behind it quickly to strike. But that doesn't mean that before you even get there you know exactly where to strike. The point on his body that really will be that weak to such extreme damage. You'll have to see it, study it, line up your blade, realize it's off by a few degrees, adjust it, hold your hand perfectly steady, and drive it in. Think of it like parking a car. Imagine you have a car that can travel hundreds of miles an hour, and has the ability to stop on a dime. This doesn't mean you can parallel park in an instant. You can zip along, get to the spot, and stop instantly, but the process of parallel parking remains difficult and precise. Just because you could zip forward at another hundred miles an hour and stop a fraction of a second later at the right spot, when the "right spot" is a matter of a few inches between "not far enough ahead to make the turn" and "too far ahead you've just hit a car," doesn't mean your own only-human reflexes are able to react that quickly to a car traveling 100 mph. You will need to take the time, go slowly enough for your own mind to process things, and park specifically.
This is the sort of think you're talking about your character doing. It's not like running 100mph, stopping for a moment and then hitting a button, then an instantly later traveling again at 100mph. Once you get into position, lining up the actual perfect strike to deal as much damage as you're talking about relies almost entirely on your reflexes and mental speed, which will be no faster than your opponents without feruchemical zinc.
The mechanical comparison is as follows. Imagine that you're able to get behind your opponent through some means other than feruchemical steel. He knows you exist, he knows you're an opponent, he knows you want to hurt him, but you've successfully managed, in this moment, to get behind him. Without feruchemical steel, in this example, you'd be granted a condition, one of two of which you'd need for a Circumstance. In other words, you'd be granted half a die. Yet you're claiming that by achieving the exact same feat, simply getting behind someone via running fast rather than sneakiness or a steeljump, you should be granted, not a small bonus, but up to 40 charges of speed, enough to give you ten dice, two free nudges, and a flat +2 to your Outcome, all without any penalty whatsoever because apparently you're using absolutely none of that speed in the actual strike.
Once you're behind someone, if your reflexes aren't any faster than your opponent's, the advantage is on your foe. You have to line up, no faster than your own brain can normally process, the absolute perfect critical shot that will deal absolutely insane amounts of damage. I gave blood a few weeks ago. The woman who stuck the needle in, whose actual job is to find veins on people and stick needles into them, took a solid minute to apply contrast to my skin, do a few squeezing tricks to get the blood flowing, find the vein, mark it with a pen for guidance, and then puncture it with a needle. Granted, it would have taken her less time to simply cut the spot, but it wouldn't have shortened the time down to a second. Yet you claim that if you see someone's back and have no faster mental processes than anyone else, you'll be able to locate a weak spot in armor you're seeing for the very first time, get your blade right there, angle it perfectly so that it not only goes in but does so much damage on the inside that it will do twice as much damage as a sword-strike to an unarmored person, and you're doing this all in less than the time it takes the other person to think, "Hey, this guy ran behind me, I should turn and face him" because the thing is, the other guy doesn't have to perform surgery the way you do. All he has to do is shift, ever so slightly, and suddenly you're starting your calculations from scratch, and NOW you have to hit a moving target.
There is an upper limit to how much "safe" damage you can do via precision with physical speed but not mental speed. At some point, you have to start relying on "I'm swinging this dagger so fast it simply gains the force to penetrate your body and ruin what you've got going on inside your organs". When it does, all sorts of bad things can happen to you. That same energy is being transferred back into your body. Depending on the specific weapon, there's a lot of other ways to hurt yourself by swinging or thrusting something that hard.
In conclusion, battle is not a situation devoid of thought. You are fighting an opponent who will be trying to react to you as quickly as possible. Without faster mental speed, you will quickly reach the cap of how much help simply physical speed will do for you.
I therefore propose the following. Any "charges" spent individually, i.e. to increase your dice pool up to its maximum of 10, are considered below the cap. You're moving fast enough to make it difficult if not impossible for your opponent to react before you're done with your action, yet not so fast that you risk harming yourself. However, once you start spending it in chunks of 5 and 10 to increase Nudges and Outcome, you're beyond how fast you can be and maintain the requisite precision; remember that your opponent is not a road you are running along what isn't going to react to your footfalls, nor is he a ring of keys that will present each key one by one without trying to trick you. Via nudges, Feruchemical steel can deal an additional damage for every 5 Charges spent. I suggest that for every 10 Charges spent to either gain Nudges or to directly increase the Outcome, the feruchemist must take one damage, as it is assumed the feruchemist has to rely on speed translating into force to achieve these remarkable feats. Any feruchemist willing to restrict himself to the rather reasonable conditions of only gaining the maximum dice for a pool and purchasing a single additional Nudge is still under the cap and will take no damage to himself from extreme speed.