This is all a result of me implementing sequences for .mid file playback and not thinking about how it would be used outside of that context. 
A couple of related issues:
I've got a fix in the queue that changes the tempo to a floating-point number. In the mean time you can increase the tempo resolution by using more steps per beat (.mid files usually have 480 steps per quarter note, e.g.) and scaling up your tempo to match: To get 80 beats per minute at 480 steps per beat (and 1/60 minute per second) you'd use a tempo of 80*480/60 = 640.