Question: If you slow down the GPH flow of the Aquaclear, does that constitute for the water to have more exposure time with the filter media or will it be the same as when you have it at max GPH flow?
The aquaclear is designed to recirculate the extra water rather than just restricting flow, so yes, that water passes through the filter media twice.
It should be mostly clean at that point though, the major benifit is less wear on the motor. You'll get most efficient cleaning of the tank if you're filtering more of the really dirty water rather than super-cleaning a small amount. Do you have a need for low-flow in your setup?