Thats been around for very long time...
I first installed the MP top in 1997 or 98 or something like that. I installed it with a 5% OD pulley and never really got the chance to experience the initial power increase from that part alone.
I do have to say that the performance did increase over stock and a lot of people rave about MP's top as well as ESM's top. Both are great parts, ESM is cheaper, but MP has pointed out areas of comparason that claim to improve over ESM.
I own the original casting as well as the new one and I must say it is a premium part and made very, very well.
www.magnumpowers.com has a detailed writeup on the part as well as other very very very nice toys.
In my opinion, a pulley, top, bigger throttle body and Magnum Powers inlet plenum is a great place to start if you have the cash.
But a pulley and the top will give you a nice, noticable performance gain.
However, the cardinal rule with SC's as far as my experience goes is UNCORK THE EXHAUST first and then get to the engine.
Dont go nuts with the exhaust, but happy gains are made with nice exhaust mods that dont break the bank and bolt on parts.