Hard to assume or be specific unless using the exact same pipe on every head but generally speaking if it was a 1-3/4” pipe being used for his numbers, and a 1-7/8” pipe were used what %increase would you think be seen?
for my info I don’t use a pipe because like you said it varies with size so why bother. It does make your flow numbers look better. I know I’ve said it 100 times but honestly I really don’t care about percentages or flow numbers on the exhaust side. I want to see 325 plus FPS airspeed, be a nice quiet port, and I know it will work. Piston travel is helping so if it’s fast and shaped right, it will work.