So i mostly fire from out of the enemies visual sightrange, the UFO extender accuracy (as far as my understanding goes), would still allow this tactic but the probaility of hitting with snapshot would be highly decreased, using aimed shot would work, but the gained exp would be to low, so no go for me.
The tactic still works. From what I've seen, auto shots get a penalty of 2% per square more than seven or so (Which is really close - it reduces auto shot to point blank only, basically), snap shots get a penalty of 2% per square at a little over sight range (so just out of sight, and snap shots work fine). Aimed shots have no limit.
You use aimed shots more than snap shots at longer ranges, and as it gets more important to hit more, you get to use snap shots more.
It doesn't matter for xp, though - the primary action needed to get accuracy xp is hitting the enemy - not shooting at it. So it's the amount of hits that counts, and that only changes as you start using stronger weapons.