This could be caused by anything, but the baseball like curve sounds like a spin issue. Aside from making sure the BB is spinning perfectly backwards (and not lop sided), which it sounds like you already did, there are few things it could be and some are easier fixes than others.
Check the crown. The last thing the BB touches before it enters into it's External Ballistics phase is the tip of the muzzle. If there is even the most minute ding, nick, or irregularity at the end of the barrel, it won't matter what components you have in the rest of the gun.
Another thing could be the inner barrel itself. Are you sure it didn't get bent or warped somehow? If there is a slight curve in the barrel, the BB will rub more on a spot and that'll throw of it's spin in flight too. You may also want to the check the cant in the rifle. Double check and make sure everything is lining up well.
How long is your barrel? I'm not entirely sure how this will relate to airsoft, but in real steel shooting you have to make sure the bottom of the bullet is boat tailed so the left over gases and pressure doesn't throw of the trajectory by passing unevenly around the bullet. Since BB's are round, this might not be so much of a problem, but it's a possibility that if you are pushing a lot of air down the barrel it could still affect the BB after it leaves the barrel.
Does the barrel extend all the way to the end of the muzzle? There's the possibility that the BB is striking something after it leaves the barrel?
Harmonics. Whatever you have for barrel spacers between the inner and outer barrels you will want to space evenly. Consider even adding in more than they usually have in stock rifles. If the inner barrel isn't as fully supported as it could be, it may bow under it's own weight.
Did you get a lemon barrel or hop-up unit? There is a possibility. Try and find some way to measure and discover whether a piece may be malfunctioning. Has it ever worked properly?
Finally, is it your technique? I've seen plenty of school qualified snipers bitch about equipment that sucks until later they've discovered the equipment is so high-end that their poor trigger pulling technique now shows. Have someone else shoot the rifle.