All true. I'm basing my seat of the pants belief though on the fact that my FB winds out much quicker than I recall my X ever doing. It might be due primarily to a tactile sensation though, as being a harley my FB feels like there's a whole lot more going on at speed than my X, which seemed much smoother. This makes me feel that the top speed of the X would be higher than that of the FB.
I do recall that HD had to purposely add in more vibration to the M8 during development because early testing and reviews by test riders that felt the motor was too smooth, for an HD. And it's the vibration that makes me feel like the X has a higher top end. But then again, I'm far from being an expert. I did have the same feeling about my Deuce too, at freeway speeds.
I did have a Stage 1 and 2 done on my bike in August. All HD Screamin' Eagle parts including the tuner, except for the air filter which is K&N.
M8 Fat Boy S Bonneville Salt Pearl