I have a Hatsan 135 in .22 and love it. It consistently shoots 1000+ FPS with H&N FTT 5.55 head pellets. I use open sites and shoot a 0.4" taget at 13 yards and always hit the target. This rifle is not for indoor home shooting with it's 30+ FPE power. This beast is ment for hunting or a real indoor/outdoor range. It feels and shoots like a bazooka. I wouldn't dream of putting a scope on this thing as it's going to eat them up and render them useless unless you buy a real expensive one. I can't see spending $400 on a scope to put on a $220 rifle. Just my opinion. Anyway, for the money, this is one of the better deals out there for quality & price. I'm glad I made the purchase. I felt mine didn't need a tune out of the box.
Have you tried a pellet with a slightly larger diameter? The Hatsans tend to have a loose bore, and larger diameter pellets can make a huge difference.