Doing a porting job can drastically change your engine's powerband characteristics. IMO, it's one of the most advanced things you can do in the gas r/c world. Most people don't do it right and they won't admit they did it wrong, much less realize they did.

From what i've heard, hand modified engines from people who know what they're doing tend to be VERY good engine and a night and day difference from stock. I wouldn't recommend anyone doing a porting job unless you have EXTENSIVE knowledge of how the engine works as well as the right equipment to do it. Also, be prepared to scrap the engine if it doesn't run properly.