So I'm about to run a barra engine in a speedway class that requires me to use a factory-ish manifold (externally stock with unmodified throttle)

The current engine (sohc) never sees below 3500rpm, and the barra will be revving harder. I don't need the long runners.

How much could I stand to gain by porting the manifold (including milling the top half to remove the long runners in favour of plenum volume)

Thanks
Ben