The vast majority of the wire in your house is 14/2 rated at 15A. To go to 20A you need 12/2 which is a lot more expensive and no one installs this just in case. You cannot simply upgrade 14/2 wire to 20A, any electrician doing this should have his license revoked. As someone mentioned, you wouldn't want your insurance to be denied in case of a fire. Here is my big problem with this, that Dewalt blew a 15A that means it is running border line 15A or more. If you had a 14/2 with a 20A breaker and your load was just a few amps it would still not be legal but you would never see issues, this set up will be pushing the wire rating regularly. Dubber, for your own safety and that of your family, please return the breakers to 15A and run a separate 20A, 12/2 circuit to your compressor. BTW, you also need a special plug rated at 20A, those regular $.99 plugs are rated to 15A.
I doubt the compressor was the only thing on the circuit, so that's more than likely the reason it popped a 15A breaker. I agree with you though and even mentioned to him to run a separate circuit, especially if the panel is in the garage and overhead access is available it's stupid easy to get done.





. Your GLI looks clean and pretty killer with those wheels on it, I probably would have done the same thing.