If you have the m performance package you could get a m240i diffuser and run tips out of each side you could use either 3.5 or 3 inch tips whichever you prefer. If you want to keep your stock diffuser you can run 2 2.75 inch tips out of the one side. Can probably find some good cheap ones on ebay for you to buy. Here are a couple threads talking about exhaust and tips.
https://www.2addicts.com/forums/show....php?t=1158151
https://www.2addicts.com/forums/show....php?t=1095598
And if you still want to use the stock ones that should work the shop can just cut out the muffler and weld a straight pipe in while keeping stock tips I believe.