I dont think you can measure it like that. The 240gflops from Xenos is not equal to 240gflops from these new GPUs in PS4/Durango. The architecture is much more efficient now, so the flops are not going to be directly comparable, at least in real world performance. Also we dont know how it would be potentially set up in Durango architecture wise, the 360 SoC.
It also doesn't seem like the easiest thing to take advantage of(maybe a programmer with more knowledge coud shed some light here), a PPC architecture combined with x86 seems like a headache, and therefor you might not be able to take advantage of it as much as a x360 by it self would, could equate to a lot of wasted cycles(pure speculation here). Seems like a lot of work for little gain, I could also see only a handful of devs actually using it. Hell devs might not even have access to it. It might just be for BC/OS/Kinect, as this would make a lot more sense.
I still think the whole thing is BS based off of cost reduction down the line alone. If the 360 SoC is required for anything other BC, you would never be able to remove it.