This is a tough topic with much debate. Shaft drive definitely costs power over a chain or belt, but how much I think is negligable and difficult to detect for everyday riders. My last bike was a shaftie and I loved the fact that beyond changing final drive oil, there is no maintenance. If you were going to competition drag race or something it would be more of an issue. As far as numbers go, I have seen everything from 4 to 8% up to 20%. But its hard to say. Is this a deciding factor or do you already have a 109?
I think, based on Lamont's dyno figures vs. claimed HP at the crank, it was about 14%, which is a little less than the VTX. That is a loss of about 17-18 hp on the M109R. Given the convenience of the shaft drive, I can live with that.