I like the idea, but I think it's a little cheap, especially if you can get 2 per special slot.
The ensorcelled rule makes all the difference in terms of performance, even DE reapers spend a lot of their time hitting on 5+ and they're trained artillery crew. This would mean that the bone thrower could pick off lone wizards etc. with 5+, especially with D3 wounds. Maybe if it was just +1 to hit, to balance the skeleton's BS, and not ignoring modifiers.
I think they should be more expensive, because the army weighs ranged equipment/units more highly than other armies, given the way it plays.