I've seen it numerous times!! But it may not be the problem.
And having the lugnuts not correctly torqued would not be tempermental w/speed either.
Wish him luck getting it straight!
There is no proven fact that the side exhaust robs horsepower. Even on a dyno the systems were almost identical. Within 1-2hp. Look at the stock exhaust on a GT and tell me the bends over the axle don't rob some horsepower?
I've heard alot of good things about MMR. But the 600 comes with a cast crankshaft. If it was my money I would toss the extra money towards a forged crank. Your only talking about a differnce of $600. Well worth it in my opinion.