cutting the bend

I have a friend with a 03gt m5 who has a mac cai on his car. Ever since he's gotten it the cel has been tripped a few times. He finally got fed up with it, so I took a hack saw, and cut the bend off the cai to make it into a strait shot. After reinstalling everything, I took it for a test drive and noticed that the car pulled much better in the upper rpms, and the cel almost immediately turned off. But now in 1st gear the car seems to bog bellow 4,000rpms, and doesn't really start to pull until second gear. Also he's probably going to have to get a heat shield for it, because after it gets hot under the hood, the performance drops significantly. What do you think is causing 1st gear to bog? and what can he do to fix it?
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