Well, we are where we are. Specnas come with an M90 spring[*]and I was seeing around 0.83J ("300fps") out of mine today with the M90 in it. 90m = 295 feet, so that's acutally about what I'd expect. Fine for CQB, not so great for woodland (only Tokyo Maruis go further with less energy

)
Hop
should effect energy. If it was shooting at the same muzzle velocity with or without hop then it sounds like something was wrong there, and it's now actually applying hop - and you won't need much to lift 0.2g. Although 0.58J (250fps) is pretty weak, I'd want more than that for outdoors.
Given the quick change system in the Specnas, and that the air seal could be better (reportedly and from experience) I'd be looking at looking at throwing an M100, 105 or even 110 spring in there. Or sorting the air seal, but I couldn't get mine perfect with grease or new o-rings, and a spring is cheaper than changing any other part.
[*]As well as an M120 which isn't much use for the UK - too much for an AEG, likely not enough for a DMR.