Advance Design shocks (http://advance-design.com) are in the same price range, $400 a corner. They look on paper to be closer in concept to high-end shocks than to Koni Yellows (which appear to have less volume than the stock shocks), but I've heard mixed reviews; some people swear by them, while some people revile them. I am not aware of anyone using them in a Stock class autocross S2000.

Mitch Fagundes (6th at Nationals last year; I don't think he's on this board) is using Bilsteins that he had custom valved. I haven't seen adjustable Bilstein conversions (other than the late, somewhat lamented ShockTek), but some argue that the monotube Bilsteins are better shocks than the twin tube Konis.

Steve

 

Oh, and for $500 a corner you start getting into single adjustable Penske / Moton / JRZ territory.

Steve