Let me throw in a dyno test result on theory, the first motor I dyno tested was a stock stroke 413 M.W. pump gas motor with the stock unported 1962 M.W. heads with all new seats and stock size replacement valves, same heads, intake and carbs. as on all the production 1963 M.W. motors.
The testing was done at Pettis Performance back in the mid 1990s on their new DTS engine dyno.
We started off using a set of Hooker 1 7/8 diameter 42 inch long primary pipes A body fenderwell headers with the 3.5 inch collector dumping into a five inch dyno exhaust pipe straight into the outside air, no muffling. We made several pulls after the cam breaking in and several more different pulls testing ignition timing, valve lash and maybe some jetting, this was long before O2 wide ban sensors so we where using EGT readings and BSFC as our tuning aids.
Bob Pettis made up a new set of dyno headers while we , his Son Jason and I, where doing this testing at night after I got off from work. He made his dyno headers with 2 1/4 primary pipes 25 inch long and dumped them into a 4.0 inch collector
Jason and I talked about the potential loss of HP and torque on this 426 C.I. low compression(9.4to1) pump gas street motor from these large headers, those stupid parts, that motor and headers, fool both of us
It picked up the average HP and torque noticeably
Theory is one thing, real world is another thing, so test, test and test some more with a open mind and look hard at all the results