After lowering the requested difficulty from 128 to 64 my stale rate went up from ~0.5% to ~1%. Does this make any sense?
Halving the requested difficulty you find and submit twice the shares in the same time. So if you submitted X shares in the "stale time", now you're submitting 2X shares in the "stale time" (of course X and 2X aren't exact values: we're speaking of averages). But now every share is worth (about) a half too.
The stales are always "prevhash-stale". Are those stales payed?
Sure. You can see it in your account page (please click on your bitcoin address in the main page), looking at the btc column: even if a share is prevhash-stale, nonetheless it is payed.
Obviously other types of rejected shares (for example duplicated shares) aren't paid - and you'd see 0.000000 in the btc column.
I'm wondering because paying stales would almost eat up my fee (1.5%).
Don't worry. The lower is your requested difficulty, the higher is the fee. So: if you lower the difficulty, the pool gains more, no matter what.
