The H.O. kit is the S-trim kit (same thing). When you buy the S-trim kit you get alot of goodies you don't with the A-trim kit. The A-trim kit + $685 upgrade is not equal to the S-trim kit - you still would have to buy more stuff to equal the S-trim kit.
The S-trim kit includes high flow in-line pump, MSD unit, and I think a reversion valve. Visit
www.vortechsuperchargers.com for full list.
Procharger also offers a great 9 psi kit about in your price range too, just to complicate matters.