I remember seeing a relatively extensively modified Prius for autocross (think rollcage and harnesses) quite a while back. I'm not able to find it anymore though.
There are rear bars available from TRD, and junkyard Matrix parts will fit. For autocross you probably want to increase rear roll stiffness, not the front, which would make the understeer worse. I put a bigger rear bar on my Impreza, and it really improved autocross handling. Haven't done anything on the Prius except BT rear chassis brace, and BFG G-force sports which made a HUGE handling difference (wet and dry), but cost 6-7 mpgs . Haven't taken the Prius to the autocross, the Impreza is so much fun; it would be kinda anti-climactic to run the Prius, unless there were lots of others to run against.
Sigma's got the TRD Sportivo suspension package that gets you (I think) both higher spring rates (it might help control the body roll) and a slight drop.
SCCA autocross rules for stock classes only allow the following mods: Front sway bar, shocks (not springs), tires, wheels (must have same offsets), cat back exhaust, alignment, removal of all lose or temp tied down equipment (spare tire etc...) & training for the driver. Do more than that and you are racing with the big bore boys & girls. Stiffer springs up front should slove the sway issue. I already have Touring set up, which is what TRD is offering. This has not solved the oversteer issue we experience on highway.
Bullitproof Auotmotive has found a stiff front sway bar from Cusco. Being a JDM part it will take 7-14 weeks before arrival. It has been ordered.
Putting a rear bar moved my Impreza to STS class, not stock, as you stated, but the time I spend on the autocross track, what, 2-3 minutes total vs. all day on the road, made the change worth it. I just autocross for fun, and need to drive everyday. Bang for the buck - get the rear bar.
Touring TRD is the rear bar available. It came with the car. Agree on improving FWD cars with a rear bar. But, I am after more understeer.
