Having owned both a VTX 1800 and an M8 I would have believed, and have stated the opposite. That he would beat you off the line but you would reel him in and pass him in the long run.
How far did you two go before having to slow down?
Off the line is kind of subjective given that different riders may very well have much different capabilities of getting a good launch. I'd be almost willing to bet that you could do a quicker launch on an M8 than you can on your X.
M8 Fat Boy S Bonneville Salt Pearl