I believe its squared, but physics drove me from physics and into business so I wouldn't bet more than a $1 on it. You are not getting push, you just aren't having to overcome air resistance, so your power increase to speed increase is linear (rather than exponential) when in a perfect draft. I think a pro cyclist could easily hit 60 mph in a draft so long as their gearing is appropriate and the vehicle gave them an appropriate lead-out (and they are going downhill ).