head comparison

I know the bottle neck in the motor I will be running in my dart is going to be the 915 J heads. I did step up the valve size when I had them redone.
So heres my question:
What does a 915 J head flow stock? how much more with the larger valves. And how much would I improve the flow if I swapped them out for a set of x-heads? What do X-heads flow?
Thanks
Rodney