I'll just reiterate the STS AW11 suggestion, although I'm admittedly biased (love mine). Like any car that isn't "the one" to have it's going to be an uphill battle against the lighter CRXs and Miatas.
However, like Brotus7 said, there's been some interesting info/development in the MR2 community dealing with suspension setup thanks to our favorite curmudgeon, Steve Hoelsher (sp?), who is a past DP Nats champ. Following his swaybar-less methods a few board members over there have been knocking on FTD locally. The Macstrut setup is limiting, but can be overcome imho.
Besides, they kinda meet your criteria of being cheap and a darkhorse of sorts. There are a few other classes that have potential as well if you decide to get more serious. I think 4agze swap into the lighter and t-topless 85/86 model would be fun in CSP, and Steve's already proven they're competitive nationally in DP, although with a heap ton of very well thought out development. Also, Zipty on this board (and mr2oc.com) is in XP, although I'm unsure of how he's doing.