There's been a few threads on this in the past.
The biggest benifit seems to be improved throttle response. Some guys have reported gains, while many others have reported little or no gains in HP.
I know two guys locally (L99s) that made very little (not really measureable) on the dyno, but they swear by the improved throttle response.
Here's a write up by Texas Speed. They made about 3hp with a ported intake and throttle body on this car.
http://www.camaro5.com/forums/showth...xas+speed+dyno
I think the gains will be better on the LS3's vs L99s.