The clocks on these(mine anyway) is quite different from anything in the past, it takes a bit of tweaking, increasing the core/mem without finding the sweet spot for both will not result in better or the best performance.

For example
595-869=9899 in 05

compare to a tweaked 570-902=9905 in 05

I have also found that not running the correct core/mem tweak will result in throttling, I use 03 first couple of tests for a rough guide on setting the correct core/mem ratios, if it passes wings of fury, it is usually a quick indicator that I am on the right path.

My card is from the first batch XFX 520/1500 un-modded