I play drop 'A' on all my guitars, I use 009-054's on all my seven string guitars and I dont seem to have any problems.
I hammer the trem all over the place and they stay in tune fine, I think the secret is regular setup and just keeping an eye on your guitars.
I was thinking of dropping lower at some point, but dunnno if I can be arsed with having telegraph pole wire on my guitars for strings!
