PES' wiki page on the mod:
Directory:Gadgetman Groove - PESWiki
It appears that basically, all that's being done here is adding approx a 3mm deep groove in front of the lower part of the throttle plate, and behind the upper.
So what's really happening is a trick way of telling the ECU that you're at X throttle position while you're allowing the same amount of airflow as Z throttle position. Other than that, I can't say for sure. I may give it a shot on the spare TB I have for my wife's Saturn, though. It takes about 10 mins to put the TB on, so that shouldn't be a problem.
EDIT: OF course, if I do this, and there is no change/negative change, the guy's just going to say I did it wrong. It won't be an extremely definitive test, either. I'll see about using my datalog tool to check MAP and TPS values, etc.
EDIT again: Wouldn't you know, I can't find my Dremel. Gonna have to buy another one, again, it seems. O well, excuse to go to Harbor Freight.