I have been using NAP Spitfires for the past 4 seasons, and have had excellent results. You may also want to try out Rocket Broadheads, as they seem to be the most popular mechanical broadheads with the members on this forum.
As far as speed is concerned, I would like to be shooting at least 250 f.p.s. before I used mechanicals. Thats just my opinion, and I am sure you will get different opinions from other members.

Good luck