Just a thought, but there's a 1500 "fix" from the VROC site called the "Piver Punch". Take a long rod, stick it down the pipe, and beat it until it breaks through the inner cover. I did that on an old VX twin I had. it made modest changes to the sound, just enough to give it a bit of rumble, but not loud at all. Depends on what you're trying to accomplish.
Seems to me that there's still restriction from the goats belly. Does the 900 have one of those like the 1500? That's why I didn't do it on my Vulcan and just went with new pipes.