Two 100w chips reduce performance per chip 50% add together 50% + 50% = 100% you're still at 100w and double the performance.
Sorry, but what are these numbers you're working with? Are you inventing them out of thin air? And what's the relation between the different numbers? You also seem to be mixing power and performance? Remember, performance (clocks) and power do not scale linearly, and any interconnect will consume power. You're making this out to be far simpler than it is. Other than that all you're really saying here seems to be the age-old truism of wide and slow chips generally being more efficient. And, of course, you're completely ignoring the cost of using two dice to deliver the performance of one.
The node reduction itself from 6nm down to 5nm is what 1/6? across two chip dies which works out to 1/3
What? A 1/6th/16.67% area reduction from a node change will be 16.67% no matter how large your die, no matter how many of them you combine. A percentage/fractional reduction in area doesn't add up as you add parts together - that number is relative, not absolute.
It's absolutely possible that an MCM approach can allow for power savings, but only if it allows for larger total die sizes and lower clocks. Otherwise it's no different from a monolithic die, except for the added interconnect power. And, of course, larger dice are themselves a fundamental problem when per-transistor costs are no longer dropping noticeably, which is leading to rapidly rising chip prices.
he other good aspect is heat is spread out more between two chip dies which is better than a one chip die the size of 2 all condensed in one spot. It's much better for the heat load to be spread apart and radiate more to the cooler.
Again, this isn't accurate. A GPU die has its heat very evenly spread across the entire die (unlike CPUs which are very concentrated), as most of the die is compute cores. Spreading this across two dice won't affect thermals much, as both dice will still be connected to the same cooler - it's not like you're running them independently of each other. Assuming the same power draw and area for a monolithic and MCM solution, the thermal difference between the two will be minimal. And, crucially, you want the distance between dice on package to be as small as possible to keep latencies low.
That even reduces stress on the VRM's that have to power the fans for the GPU. Something interesting is if a AIB's were to ever put a fan header on the side that could be plugged into a system header instead shifting more stress to the MB VRM's and off of the GPU's VRM's given they can consume a few watts.
Fans generally run directly off 12V and don't rely on VRMs on the GPU, just a fan controller IC sending out PWM signals (unless the fans are for some reason controlled through voltage, which is rather unlikely).
You've gotten the whole thing backwards. The 6700XT is a very well optimized chip and runs at 2.5+ghz not because they were pushing it hard, but because the silicon was good and could deal with these clocks without any issues. 6800 was produced from (badly) failed Navi 21s and thus its clocks were the lowest out of the whole RDNA2 series.
Sure, ideally Navi 22 would have been a 60CU design and thus with only 25% cuts applied to computation, i.e. the same as the cuts it got to its memory and IC subsystems. But I imagine when AMD had to plan for how many chips they can get from their 7nm wafers they underestimated their yields - or just decided to err on the side of caution - and we never got a symmetrical trimming down the product lineup.
Idk, I think the truth is somewhere in the middle. Both chips have distinct qualities and deficiencies. The 6800 is fantastically efficient; the 6700 XT gets a lot of performance out of a relatively small die. Now, the 6700 XT is indeed rather poor in terms of efficiency for an RDNA2 chip, but it still beats out the majority of Ampere GPUs, so ... meh. (The 6500XT is another matter entirely.)
I still can't wrap my head around AMD's RDNA2 segmentation though. The 16-32-40-80CU lineup just doesn't make sense IMO, and kind of forced them to tune the 6700XT the way they did. 20-32-48-80 or something like that would have made a lot more sense. It's also weird just how few SKUs Navi 22 has been used in overall.
