I would love to learn more about that extra chip. Is it an off-the-shelf IC or an ASIC?
I don't have any X100 boards where I can lay hands on them, but from what I remember, the chip was labeled with an Apple part number, but it's entirely possible that it was a rebadged CPLD. I checked my notes, but I didn't record the part number. Electrically, it's going to sit between the RAS and CAS lines and the 0802 chip which is one of the big ASICs in the chipset. And, of course, the 6100 lacks the chip in question. I think that the Q840AV does the same thing. It certainly behaves the same way, as far as memory operation goes.