I fully agree, I have both TrackIR 5 and Eyetracker 5 and the latter feels sluggish somehow. And I turn off the eye tracking and just use head tracking. TrackIR is entirely lag free I would say, much nicer experience.The head-tracking (with TrackIR in my case) is much more responsive. My guess is that the raw inputs from the sensors needs to be smoothed out to handle the involuntary eye-movements; resulting in lag being introduced in the response to your eye movements.
However! The Eyetracker 5 just sits under your screen waiting, you don't need all the extra crap on your face to make it work. It just works.