The walther has an FAC stroke length and an inefficient transfer port. If you could alter the transfer port length on the walther to 6 to 8mm with the same spring/stroke/preload the gun would be doing 16fpe I bet.

One way of improving the Walther may be to alter the barrel so the barrel tube stocks thru into the breech face. Bore down into the breech face leaving around 6mm or so of breech face for the transfer port.