Technically any altercation to any part of the car shouldn't pass inspection. Even a ford racing muffler on a ford car (my buddy has a setup like that thats why i mention it) shouldn't pass. As for emissions, it all depends on the car. My toyota could drive around with like no muffler at all and it would pass (not really but damn near).
To op, if you want better gas mileage just maintain the car, give it a tune up, clean the injectors, add some fuel additives.
Driving 100 miles a day, you WILL REGRET a loud exhaust.