yes, they say it hurts performance. i wouldn't do it. when i see a 4-cyl car around with duals, it just makes me laugh. kind of like putting "type-R" stickers everywhere or cobra emblems on your v6 mustang.
theres a couple of people who have them with their EVO kits....i think one is a dual can set-up and another is a one muffler with two exit pipes with 2 tips.....I don't think anything is wrong with that one.
I'll have to look around on pics folder to see if I have some....I think some of them looks really good IMO.
Two Straight-Exiting tips from a single muffler should be fine. What really loses performance is the set-ups that have a "T" to make the tips exit on the left and right a few feet apart. That's 2 additional kinks in the system.
Hell, why not a muffler w/ 8-10 tips coming out, sort of looking like some of those rocket launchers they use on the sides of helicopters. The new "Spirograph" exhaust tip...
I also think that dual pipes can look great, and even on an inline four, can you say S2000? or MR2? So what if it adds a couple of pounds, so does my sunroof, but I still love it.
People that put 17's or 18's on their cars better not rip on dual pipes either, because they sacrificed performance for looks, and I bet 18 inch rims will hurt 1/4 mile times worse than 15 lbs of extra pipe and a t-section. (And I know 17's or 18's might help handling, but with the same width tread, a 15 or 16 will do just fine, and might even be better, in an autocross.)
The only way that duals would hurt the performance is because you would lose back pressure in which case yo would lose low end torque but with more out flow in the exhuast you gain in the higher rpm's. Just depends on what you want out of them. I wanted duals on my car for the looks but I can't come up with a way to keep my low end torque, so I don't think that I'm gonna do it.
The "T" of most duals, (Not just dual tips on a muffler) would Restrict flow, and Increase backpessure...Probably TOO much for benefits of any variety.
BTW, the 17's being lighter than the 16's is definitely a Plus, but it's partially negated by the fact that a greater amount of the weight that it is trying to turn is located farther from the center, which results in a loss of torque at the wheels, since the force of leverage is on the side of resistance. It takes more power to spin a larger diameter wheel of equal weight up to speed.
The fact that they're lighter overall helps a bit though.