I gain about 1-1.5 mph with my TT bike over my road bike. I wouldn't gain anything close to this though by putting aero bars on my road bike because I wouldn't be able to put out the same power output for any length of time being in this awkward position.

For example, I am in almost the same body position on my road and TT bikes just rotated more forward on the TT bike. The geometry of the TT bike allows for this more forward rotated position without changing my overall body position much (as if I was on my road bike and someone lifted up my rear wheel a foot or more). Aero bars on a road bike would be a completely new and less efficient position.