Seems most guys are claiming around 330 340 CFM on stock heads, some claiming as high as 400 ported. Also seems that a 392 SRT8 can make around 580 HP without pulling the valve covers with the addition quality long tube headers and a tall M1 style intake and big throttle body or carb, thats why hotrod magazine didn't bring one when they did a chevy and ford motor up the same way, they even got cam upgrades and still would have lost to the 6.4. Dynos and drag strip numbers both seem to be backing this up. I am sure the stock intake is some of the hold up but not all of it, on the other hand you port the heads, big cam, more compression the stock intake will be an even bigger hold up.