The stock port at the head can be opened up 20-25% by matching to the stock gasket. Along with un-shrouding the injector boss, this is how much flow should be improved to the valve. With the heads we do, port matched, bowl work, perf valve job and ported intake, local GN's run low 11's. Same car/combo/boost before ran high 11's.
Hard give specifics on how much more power a ported manifold would give as we have not done just that, but will certainly help.