There is one problem with a variable-speed lift chain: the anti-rollbacks.
The way the anti-rollbacks are designed, when the train is rolling forward a wheel creates a magnetic force that keeps the anti-rollback "dog" from dropping down into the rollback teeth (or something along those lines). If the chain were to stop or break and the train started moving backwards, the dogs would drop down and engage the teeth on the track.
That's why you hear the anti-rollback dogs only at the bottom and top of the lifts, because the speed isn't constant there, and it affects how the dogs are held in their upright position. (the curve of the track might also be a factor) Lowering the lift chain speed would affect the dogs' ability to stay in the "up" position.
This happened a few years ago on Superman: Ride of Steel at Six Flags New England. It uses a similar system to keep the dogs up out of the way, making for a silent lifthill. But for most of one season, for some reason the lifthill was run at a slower speed than the designed speed, and the dogs couldn't be kept in their up position so they clanked along the teeth in the track and it was VERY VERY loud. (You had trouble hearing the person sitting next to you)
If this same situation would happen on EE's lifthill, it would make it VERY loud and obnoxious both for the riders as well as people anywhere near the mountain or across the water.
-Rob