Jester, you are right that Oblique Artilleryman SPA wouldn't add range to the LTAC, but for reasons different than you think. We should probably read these rules better sometimes as I was mistaken on some things as well, but I think for the most part we've been playing right.
As per pg 97 of TACOPS Advanced Units and Equipment, Artillery Cannons are not artillery, but Ballistic Weapons. They CANNOT make indirct artillery attacks. Instead, they make indirect attacks using the rules for indirect LRMs, but still scatter as per standard artillery rules.
So, based on that, they do not get the +4 direct fine penalty, as that is for direct fire artillery attacks, and this is a direct fire ballistic weapon when fired directly. It still scatters as artillery (just 1/2 distance), so it would need to be a GM call if oblique artilleryman's -2 scatter could apply. It makes oblique attacker much more interesting a choice as well (Or get both).