Paul, What about cutting extra grooves in a set of Corsas?(I'm sure a groove cutter could be got at Demon Tweeks or similar) After all, it's only their ability to shift water that is a problem.
Dunlop supersport race are supposed to be a very good track wet tyre that can be used in the dry also and are road legal. Deals to be had on them too at the moment if you listen to the porsche guys.