You'll be faster with less effort on a roadie than on a MTB. I recently put some slicks on my full suspension MTB and I can keep up with a 13 - 14 mph pace but it takes more effort than my commuter. I just started riding a Giant Escape 1 commuter and doing 13 - 14 mph is a breeze. I can only imagine what it would be like on a road bike that is lighter. For reference, my s-works FSR weighs in at 24 lbs. My Giant weighs in at 24 without the saddle bag. But without the lost energy lost bobbing up and down on the suspension, I'm quicker and it's easier.

I'm with everyone else on the n+1 thing. I'm contemplating getting a roadie. I tried out a Specialized Roubaix and wow, it felt fast. Not sure if I was any faster, but it felt tat way.

Of course, YMMV.