Performance Exhaust Muffler Only?

[vinylengraver]

BMW Performance exhaust for 128i is just a muffler.
It does not include a Y-Pipe and it requires the stock plumbing to be cut and clamped. It looks like using the same method it could be fitted to 135i just as easily.
Am I correct to assume that internally both PE mufflers (135i vs 128i) are identical and would produce the same sound?
I do not want the Y-pipe that comes with the full 135i PE.
Can I just buy the 128i PE, cut the stock muffler off, clamp the 128i PE muffler on and get the same result? (for ~$400 less...)
Incidentally, there is one for sale in classifieds right now and there are fitment questions asked there... but I am curious about the sound, not so much the fit.

EDIT: Never mind. I just stopped at a local dealer and they quoted me $1136 (Canadian) for the 135i PE (including Y-pipe) and $1005 for the 128i PE muffler.
135i exhaust is on special, 128i is not, so the difference here is only $100.

[Converted1]

Don't know if it helps, but I run just the bolt on rear section of the m performance exhaust. It was a nice sounds upgrade. I didn't install the midpipe as I have a cabrio and was concerned about highway cruise volume.