What kind of performance does it get in HDTach? That'll usually give you a clue if something is really wrong or if it's just a badly written program...
50 is ok with a SATA Drive ... as far as i know, DMA is a setting you can set with IDE drives, in SATA drives i think it's already a part of the controller, so it would be already activated ... In fact, the SATA controller acts as a DMA.