i got an 05 coupe under 18,000 miles 2 monthes ago. ive put a cold air intake and plenum spacer so far. i had to get a new warranty(5 year), but i'm ready to put headers/performance chip and tune it all(eventually a turbo), should i cancel it?

 

I'd recommend contacting the issuing company and reading the fine print of the policy to find out if the mods you're planning on doing will void it. If you can keep coverage on some of it, like electrical but just cancel drivetrain and engine warranty then thats an option as well.
Personally I don't think extended warranties are worth it so I would just cancel the whole thing, but if you like the extra safety net you have a few options. But I definitely wouldn't do anything thats not easily reversible without fully understanding what the implications would be on the coverage.