The original Xenos GPU chip area size is 182 mm^2 + SMART EDRAM 80 mm^2 = 262 mm^2.
PPE CPUs has chip area size of 176 mm^2. Xbox 360's total APU chip size is 438 mm^2.
Xenos contain 48 unified shader units + EDRAM has 192 pixel processors.
At 28 nm process tech,
Radeon HD 7790/R7-260/R7-260X has 160 mm^2 chip area size.
Radeon HD 7870/R7-265/R7-270/R7-270X has 212 mm^2 chip area size.
With embedded memory considerations, the best Radeon HD GCN with original Xbox 360's GPU chip area budget is either 78x0 or 7790. Microsoft selected 7790 variant.
Here is the crunch, Xbox One's chip size is a large 363 mm^2 with 47MB ESRAM. 32MB ESRAM's area size can contain another 7770 class GPU which can result in 24 CU GPU. Microsoft wasted about 1/3 of 363 mm^2 chip on 32MB ESRAM.
Xbox 360 has the latest/fast GDDR3 memory during 2005 while Xbox One has the inferior 256bit DDR3-2133 memory which should be GDDR5.
Microsoft selected poorly.
Sony did well with PS4's smaller APU 348 mm^2 chip size i.e. they maximised GPU size with GDDR5 memory. Sony did their best with lesser financial health (when compared to Microsoft's financial health).
My Xbox One spec's with the same APU chip size as Microsoft's Xbox One
- GPU: 24 CU with 22 CU active and 2 CU for yields at 853 Mhz with 2.402 TFLOPS
- 256bit GDDR-5000
If the APU chip size budget was 438 mm^2 as with original Xbox 360, the GPU could have been Radeon HD 7950 (Tahiti Pro with 384bit bus) with 28 CU and 256bit bus i.e. similar to FirePro W8000 (Tahiti Pro with 256bit bus). Tahiti could be a smaller chip with reduced 64bit DP math performance e.g. Radeon HD R9-285 "Tonga".
In terms of chip size, R9-290X is a GeForce 8800 GTX and Xbox 360's GPU+EDRAM chip budget wouldn't be able match that monster chip size.
Radeon R9-M295 "Tonga" (reduced speed clock Radeon HD R9-285) has maximum 120 watts.
NVIDIA Geforce 8800 GTX sets a new standard when it comes to large scale PC GPU and both NVIDIA GeForce Titan and Radeon HD R9-290X still follows it's foot print.
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