@Asomodai only just seen your first post. Yeah so keep a few things in mind. The FB SRS group is predominantly EU guys who have higher FPS limits so they will always see us firing "a bit low".
Air seal "should" always be great with a SBA rubber and FAST but a touch of PTFE doesnt hurt as you've found. Shoudl be able to cock it, put your thumb over the end, pull the trigger and the piston sit a good way out solidly with no leaking.
Your figures now sound really good with what you have, with a fully tuned VMP, Rapax 2J and Mousefart I am about 319 on a 0.48 so you're not missing ut on anything.
Only thing I would advise would be to grab yourself a Rapax 2J directly from his website. Easier bolt pull, consistent power and less twang. https://rapaxsprings.com/
Have a look at some dry lube on the outside of your cylinder, again eases the bolt pull even more.
Other than that, if it is at the right power and it's working, have fun!