Giant-blooded neanderthal actually sounds like a better way to handle things.
To me that seemed too similar to saying, for instance, orc leaders are all orc-blooded humans. I try not to be overly literal, but the Blood feats are pretty specific.
No, they're a mechanical benefit. How you choose to skin that it up to you (I'm currently playing a "saurian blooded" human, whose skyclutch feats are being played as the result of mystic training rather than being half lizard.
I applaud your creativity in modeling that concept. However, I'm not trying to model a difficult concept with existing toys, I have the whole toybox to place with. I don't want to grant Neanderthal leaders access to feats as though they were Giants. But if you need those feats, you can still make a Giant Blood Neanderthal because there is no corresponding penalty for not taking the second tier feat. In my opinion, Giant Blood is a sufficient tool if that's the tool you have, but it's not the ideal tool. It's not a question of whether someone is literally a descendent of giants; Giant Blood has effects unrelated to what I want to accomplish. You could totally say Neanderthals are just humans with the Savage or Strong Talents, and their leaders have Giant Blood.
Since the fluff in this case somewhat suggests there is a relationship between leadership and Size, I'm okay with the transformation aspect of the two tier approach. It's also symmetric with other human splinter rate chains which, while not significant, is pleasant. All of which to say, i do appreciate the feedback, but I'm not convinced.