i went to a muffler sho today to get a Magnaflow muffler put on my car. the guy told me to wait and ask mazda and shit before i do it. he said that it wont have much back pressure or something like that. he said that he has done a few cars with turbos and the turbo never worked right again because it didnt have as much pressure as it should. so my question is it safe to put a magnaflow system on my car? The cars a 89 GT turbo.