99% sure it's the tune - don't worry you're in great hands. If it was mechanical you'd have more signs imo smoke out the exhaust or other obvious signs like the car being unable to move Chet will be cautious I'm sure and get you squared. Have fun!!

The other thing working in your favor is that EVO's are fairly tough at running with poor tunes (to a certain degree) and unsupported mods so you could very well / more than likely be fine. Unlike a Subaru which if you change just a little bit is one step away from the grave. (without a proper tune asap)