Simply put, the length and diameter of your exhaust can directly affect horsepower and shift where in the RPM your powerband comes in and falls off....HOWEVER, NOONE'S ASS DYNO is good enough to feel the effects of the difference between running tail pipes and dumps. The power gained or lost is pretty much negligible. A 20 degree change in temperature can give or take the same amount of HP. And your exhaust doesn't change your compression. You're thinking of backpressure. People can argue with me till they are blue in the face....but the 2-3 or even 5hp difference isn't worth me fighting sway bar or tire clearance. Dumps are 80x easier when you have to drop the exhaust. I won't lie, I wish I had tail pipes....just for the looks. There is ALOT of science behind the theory of exhaust systems that just adding tailpipes.
Also remember, this only applies in an all motor application...boosted and nitrous apps. are a totally difference animal.
Bag of Ice on the intake. I've got a published book of well over 100+ dyno runs that proves this is worth 10hp after cooling for roughly 30minutes. Try it at the track....just make sure you have a towel around the base of it to soak up melted ice.....they don't like you dripping on the track.
One of these days, I will run the car on the dyno and then take off the tips, dumping it before axle. I'll see if there is any power to be gained. Typically, the more bends you have in the exhaust, the more power you lose.