4x26mm port ? Taking that down to 3.0 should give another 1-2 FP just like that !
+10mm on the stroke and you'll be way over....

with a 28mm bore, synth seal, and a 3.0 port, a piston weight around 250-270g should work well. with leather, I'd say the 237g current weight=defo too light. syth and the original 4.0mm port, it'd probably be perfect.