I think 4x is the goal Nintendo had set for themselves; seeing how much system, visual, and embedded RAM is available for the system. The PS3 has 256MB locked in for visuals and 256 MB locked for the system and used 120MB (down to 60MB last I checked) for the OS. The X360 is more developer friendly in that the whole 512MB is shared between CPU and GPU with only 32MB being occupied by the OS. The Wii U on the other hand has a total of 2GB of RAM (1GB for visuals and 1GB for system/OS), in addition to loads of embedded memory. The GPU for the 360 has 10MB eDRAM, 0MB for the PS3 and probably as much as 40MB of 1TSRAM (Nintendo owns MoSys) for the Wii U, as the developers have praised the amount of embedded RAM on that system. It'll be interesting to see if the IGN rumor does come true. They stated that the Durango is 5-6x more powerful as the X360 and seeing that they "GUESSED" correctly that the Wii U is gonna be 4-4.5x more powerful than current gen it might have some truth to it.
Some of your numbers are off
PS3 = 256mb XDR RAM + 256mb GDDR5, the CPU is limited to XDR ram, but the GPU can access both, 50mb was reserved for the OS
360 = 512mb GDDR5 RAM +10mb eDRAM(GPU only), 32mb is reservered for the OS
WiiU = 2gb RAM(assumed GDDR5) + rumored 32mb eDRAM, 1024mb reserved for the OS
concerning system power, the Wii U will likely have a RAM bottleneck unless Nintendo open up a lot of the ram reserved for the OS outside of this though we do not know how powerful the system is.
concerning Black Ops 2 because 1080p / 60fps. I doubt the resoluton is true, it simply makes more sense to maintain the 544p / 640p resolution being used for PS3 + 360. The 60fps makes sense though.