The warranty depends on the dealer...some dealers will continue to honor it since its a Comptech part others wont.
I dont believe headers make mileage go down...
2003 Mercedes C32
2003 Subaru Impreza WRX
What I miss:
2001 3.2CLS (Comptech Springs and Sways, Xephyr CAI, SSR GT1 18x7.5+45, Bridgestone S03 235/40/18, Door Sills)