Realistically, I don't expect the second jaguar unit to come anywhere close to doubling performance. A) They never do and B) Despite a decade of multicore processors, the 360 have a triple core unit and the PS3 having the trippy 1 CPU + 6 SPU for games, video games are still largely single threaded. They'll utilize a second core for some tasks and some new games can even load up two or three and use a fourth but for the most part there aren't that many parallel tasks in a game. I'm not saying the extra cores are useless but they're going to be less useful than the first 2 or 4.
I expect the situation will be better on the X1 since Microsoft's latest set of dev tools really eases the use of parallel tasks, as well as their newer APIs requiring the use of those tools. Part of the problem with the PS3 and it's Cell processor was that there weren't really good tools to work with it and it remains to be seen if they've fixed that or not (or will just piggy back on MS's tools).