310fps with a 0.2g BB is about 0.9J which isn't awful. It should be a little higher with an M100 spring, and as @ak2m4says, that's down to air seal. The seal on my Specna Arms Edge 1 isn't great, and it wanted the o-rings seen to out of the box.
Airsoft guns aren't precision engineering; they're very over-priced toys. It's a matter for celebration when you get them dialled in, but you can have fun with a sub-optimal gun - as long as your enjoyment is based on the performance in the bushes rather than numbers on the bench.
"Matey at the field" wasn't wrong about M100 being a decent default choice, but here's the thing: when you tell an airsoft tech exactly what to do, you don't get to blame them when your instructions don't result in the outcome you were expecting.
There are three pretty simple things you can do yourself to try and get the power up.
One, clean the hop rubber and barrel. You should be doing this regularly anyway.
Two, put some silicone spray into the nozzle, let it sink into the o-rings, then blow it out with some auto bursts before re-assembling.
If that doesn't help, then try an M110 spring. Specnas have a simple enough spring change system; you just need a long Phillips or JIS screwdriver to take the bolt out from inside the buffer tube. Once the tube is off, the spring guide and spring come out and go back in easily enough.
YouTube will provide videos for disassembly, and spring changing. The sooner you learn, the sooner you won't be at the mercy of well meaning mateys or techs.