Buffer washer mod. - I have tried this on a couple of Airsporters over the years

If you have a good Airsporter, then removing the buffer washer and so increasing the stroke by a few mm will make it even better, but the difference in velocity is not that significant, probably about 10 ft/sec on mine. You need to re-drill the piston head fixing at 90 degrees to the original hole (and nearer the piston ) and make sure that the cocking slot in the piston is long enough (it usually is) to allow the piston to travel about 5-6 extra mm

Pellet alignment can be checked as Phil says shooting into rags, or shooting vertically down into a full bucket full of water with a few rags in
However the best check for alignment is strip the rifle, put a pellet in the tap, close it and then with a plastic rod or a brazing rod push the pellet right through and out of the muzzle. Check the head for bruising If the tap is not correctly aligned/shimmed, then it will be obvious
John