Guy I've ridden with a few times has an Orbea Terra which he uses for road and off road rides. His had double rings up front (not sure what size) and for roads, he's got a wheelset with 28c tyres and a smaller cassette, something like a 12-25, and for offroad he's got another wheelset with 38c tyres and a bigger cassette, think it might have been 11-34. I guess he'd need to swap chains as well, but that's a 30 second job with quick links.
It certainly wasn't slow in road set-up, seemed much the same as a normal road bike really. I guess if you had smaller front rings you'd spin out sooner than a road bike with compact or 52/36 front rings, but you'd need to be going pretty quick before you hit that point.
Seems a good idea really, if you can find something with mounts for guards, it could double as a good winter road bike.