Adding something like embedded memory would be a good example, since it requires code support for any benefit, and it costs silicon that could be used for other things
Most PC GPU's are optimized for the current popular titles, a friend ran some tests to determine "fast" paths on current GPU's, his conclusion was they are optimized to run Crysis2.
The same would apply if you wanted to say increase register storage, if you get more performance increase in current games by increasing ALU count instead, that's what you do.
Now I want to make it clear, I'm not saying embedded RAM is a panacea, or even in Durangos case a significant win or in fact the reverse.
On Durango I'd take the numbers we have at face value until there is some evidence otherwise (read games), I also think that even seemingly significant on paper differences, won't necessarily translate to significant visual differences. I'd also be more worried by the reported ROP counts and overall bandwidth than ALU counts.