I know these don't get much love so how about adding in some extra feats for going pure in line with what they get at quasiclass initiation, Epic Weapon Specialization at level 40, Legendary Weapon Specialization at 60 and Paragon Weapon Specialization at 70 (80 seems a bit much to wait for this?). Weapon Specialization on its own is not much of a carrot to play one, adding the extra feats in gives a decent damage boost.
Isn't slinger issue more the lack of good, versatile bullets and thus being lacking compared to weapon buffed tanks / mage damage - rather than being a purely bad class? If I haven't misunderstood, haven't played one myself.
Edit: I mean, they still get a lot of cleric goodies which are yummy ^^
Slingers sacrifice 1d8 divine and 1d8 elemental/exotic dice to splash fighter. You could in theory gain an additional 18 phy with the spec tree. Put a paladin 1 and Fighter 4 splash - and you have a feat net gain. But:
Lose Magic dmg and KD bullets. Lose 5 DC on Blind/silence bullet (from 55 to 50) Lose CL 60 GR. BT and other spells.