Should be able to shave quite a lot off, I believe the actual speaker is at THE BOTTOM of the N900 with just a long channel leading up to that little hole, so nothing to damage underneath. However that also means a channel to aim the earwax, dust and water ingress down to the speaker. I'm sure there was a damn good reason to keep it the size it was.

I would always choose wired/wireless headset over voiding my warranty any day.