Hi Peterp, 9270 has a little faster core clock, but the main difference between them is the memory bandwidth, so if you use a kernel that requires little data but has high arithmetic intensity the difference would be small, on the other hand if your kernel is small or requires a lot of memory access you'll get some some speedup, I think you can expect something about 20%.
KernelAnalyser gives you an estimation, but real kernel performance depends on other factors like your memory access patterns.
I'm not sure whether Radeon 4890 is currently supported by Brook+, but if you're very concerned with performance, there are 1 GHz card models that could provide about 50% more speed than 9250.
