Mine averages 615 with 10 pumps, I am using an Alchemy Airwerks FT valve and piston. I am using the CPUM 10.5gr pellets. I don't think the 1377 is wimpy pistol,
It man need a longer barrel to see much advantage. For me they are nice 2-3 pump garage plinkers.
I wouldn't expect to see much of a power increase with just a FTP kit installed. a flat top piston just makes the pumping more efficient; moorepower is on the money, longer barrels give more power.