Eh. Given the tiny size of the cylinder and piston in those AEP gearboxes, I'd say that while 230fps (~0.5J) is credible, 300fps (0.83J ) just from a longer barrel is... optimistic. There isn't much volume in there to exploit.
However, I stand ready to eat my words on that. If throwing in a longer barrel can get it up there, then it suddenly becomes a more interesting proposition. You'll want to do a lipo conversion as well, but that's not rocket surgery.
[Don't start looking on TaiwanGun, don't start looking on TaiwanGun...]