OK, in my experience from Stangin'... if you open the exhaust TOO much, you will lose low end torque and your mileage will decrease in city conditions (also, likely to affect your off-road adventures ) The key is still the same principle as for what does what and how. MATCHING. Match your parts wisely and with some forethought, and you'll be fine. If you open the backside (exhaust), you may need to tweak the front side (intake) Remember, your motor is in it's most basic description, an AIR PUMP (of sorts) so with that said, apply physics properly....
Flowmasters, although, I don't like them a whole lot, cuz they resonate UP through the vehicle, and not out the back. I like the Flowtech Terminators mufflers... they sound great, they flow as well as Flowmasters, but they aren't noisy in the vehicle... my opinion, this is not a knock or a flame... we all have a preference with these sorts of things...

Anyway, Flowmasters are good mufflers...
My .02
Thanks...