Well, I can tell you what my experince on this is...The only aid I ever used is auto gears, my testing of manual was on Melbourne a while back, and I could never beat my best TT time, the closest was about .5 seconds behind. That said, I did not do that many laps and I belive that once you get the hang of it it will pay off in faster laps but it will require some serious practising.
Also, as Paul wrote getting to the optimal line around the track and having a setup that suits your driving, is a big part of it, I belive you will find more time to shave off there then on manual gears, at least that what I found to be true for me. But, again, practice is the only thing that really lets you improve your times.