[Dynamo]

that M135 spring should put you up in the 445FPS range. stock gears and motors will strain hard to pull that, and your battery will suffer for it. stock ratio gears can be used up to 400FPS with the use of a good high torque motor. anything above that, and you should look into getting higher ratio gears.
as for airseal, make sure to check the loading nozzle o-ring (most come with them these days). some brands fit kind of loose on the cylinder head nozzle tube, and other really tight. you want the nozzle to have just a bit of resistance to being pulled out. i test mine by cleaning any and all grease from the nozzle and the cylinder head, then tip the assembly down. the nozzle should not slip off the tube on under its own weight.
for the piston head o-ring, you don't want it to drag inside the cylinder. when not compressing air, the piston should move freely with little to no resistance. the o-ring will expand and seal once the cylinder is pressurized, then contract when the piston is being drawn back.
the seal between the hopup & loading nozzle should be good without modifying the tappet plate if you have the correct length nozzle. most of the air leaks around these two parts are not from where they seal. the nozzle will leak around its o-ring, and the hopup will leak around the barrel underneath it. to eliminate leaks around the hopup, use wax coated floss. wrap it around the hopup tightly starting just behind the cut out on the barrel. you don't even need to tie it, once it's installed back into your hop chamber, it'll be held in place.