I easily got 308 cfm out of my econo W2 heads with a 2.02 valve. I would have loved going to a 2.08 valve BUT I was only running a racer brown .520 lift solid cam and the heads were set up for 3/8 valves and I didn’t want to spend the money to upgrade them. Luckily I didn’t because they cracked. That 408 engine ran 9.80’s on BP93 pump gas at 2840 pounds.