So after the EU server, would it be lowered? Setting it at 512 would cause less shares going through, wouldn't it?
I believe somewhere in the 100+ pages there is a discussion about that and the higher difficulty does not ultimately affect the pool profitability.
But it causes less shares to go through. For example: I get 3,000+ kh/s, and when I monitor it at say 128 diff, I'm constantly sending accepted shares, when it's 512, I'll see it accept a share, but sometimes it takes too long that we already move on to the next block, so my hashing power during that time gets voided.
I hope I'm making sense.
edit: FWIW, I normally get over 3,000kh/s, but on this pool the most I've seen is 2,500 (i'm assuming this is because of the difficulty being at 512)