I spoke to the Renntech people at length about the C43 and they basically said that there wasn't a whole lot one could do to it. They do sell and airbox which seems a lot less drastic to increase HP than altering that exhaust system.

I drove the C43 a few times and may still buy one, as there are a few out there with at least a year left of warranty for the mid to low 20s. I wasn't wild about it but it seemed to be a fairly practical car. The 1998 is said to have transmission issues but there are fixes for it.