My 1987 300E had the original rear muffler fall off yesterday due to rusting at a weld joint. There is a significant cost difference between installing an OEM replacement muffler myself, and getting a universal muffler installed at the local chop shop. So here is my question: If I have an aftermarket universal muffler installed, will I have any significant performance or mileage degradation over putting in an OEM part? The car has 144,000 miles and I am not keeping it showroom ready....just want to keep it running smoothly and safely for another 100K! Anyone have any opinions or experience here?