Bringing this thread back from the dead...
Tried something and ordered a set of M3/M4 90mm tips from ARM Motorsports (very affordable tips).
Had to have them tack-welded in place as the OEM muffler in the M6 has a "bottle neck" design and it is skinnier in the back part closer to the muffler.
They stick out a tiny bit more than I'd like, which causes you to see some of the OEM pipes behind them (I will paint them black to blend it all in), but overall very happy with the results.
Dirty pic, but you can see what I mean about the OEM pipe behind the tips (from certain angles):
