This, I think, is among the biggest problems with SPG's. They don't benefit from buffs to artillery, or from a lot of the buffs to tanks. They also don't benefit from "all frontine batallions" from superior firepower. They sort of fall through the cracks and miss out on a lot of good things that everything around them gets.
Giving them the bonuses of line artillery would be a sensible way to differentiate them from tanks, and increase their viability. Maybe dispersed support would actually be good with +10% attack for SPG's
