head comparison

By doing some porting work on the 915 J heads how much flow can I pick up? I dont want to have to spend $1500 plus for a set of heads when I will only race 3-4 times a year. I know I have over cammed for the heads that are on it. a solid roller with .575 lift on both! Just looking for options so I know what to expect when it doesnt go fast enough! lol
Rod