before i say anything, please remember that the smallest few are the loudest bunch.. most evo owners have little to no problems....

now to address your concerns....

rust issue = covered by warranty
clutch problem = clutch chattering is probably related to its racy nature, same happens to alot of wrx's, although i dont know about the RX8.
chipping/weak paint = this is an issue with alot of cars these days, car manufacterers had to change formulas because of environmental concerns with weaker water based formulas...
voiding of warranty = depends more on the dealer, just dont expect to get warranty on any part around and including the aftermarket part, this is 100% true with ALL cars.
bad service = yeah mitsu service aint great, how is mazda? i wouldnt know