I recently had a newly broken-in Go Tech 3 port sport motor modded by a friend.
I knew the motors characteristics well before sending it to him so that I could do a direct comparison. I knew that the motor spooled up very quickly and had great bottom end snap while getting 9:22 minutes on a 125 ml tank. Unfortunately it topped out a bit too soon IMO.
I really only wanted to get another minute of run time, a little better top end and maybe a bit more bottom end. Well 2 out of three ain't bad. I got a bit more bottom snap and definately more top end but I LOST almost a minute of run time.
My point? You can't have it all. No matter what a modder promises you. I don't believe a modder can take an engine he's never worked on and give you amazing results. I think it's part science, part experience, and part luck. So if you're willing to roll the dice then go get your engine modded. I've seen some bad/mediocre results from several of the top engine modders.