I don't kwow. It have been only 5 days since I'm mining and I have only mine in deepbit, I haven't try any other. But I think it won't affect, it's the speed your GPU works, it shouldn't be affected by the pool you are mining. I supose 

I'm doing +/- 172 Mhash/s in deepbit with extra flags -v -w128 -f1. Try not to use anything that will use GPU power like see youtube, some movies, etc.. because it will down the Mhash/s.
I have it with a 5770 overclocked 925/1200.
Without oc it's about 150 Mhash/s +/-