I do not think the 5870 is bandwidth limited at all. There are many examples out there, and in this very thread, showing that 10%-12% increases in memory bandwidth is not increasing frame rates that much (1.5% - 3%). This small increase in frame rate is due to the lowering of the memory timings and not the increase in memory bandwidth. The same phenomenon happens when overclocking system memory. This benchmark shows that 10% increases in core speed is translating to a 5.5% increase in frame rate, where as the same 10% increase in memory bandwidth only increases frame rates by 1.5%.

You guys are getting marginal increases in frame rates when bumping up the memory speeds from the decrease in GDDR5 memory timings, and you're confusing that with a memory bottleneck.