The "standard" Dwarf Setup is two canons, one GT, 1-2 Organ Guns. Some folks go with two and two (Canons and GTs). Most folks don't go with 2/2/1. At least, not that I have gathered. That's a hell of a lot of points invested in stuff that's only really useful in the early game, and points that can very easily be handed over late game as Close combat begins and as most armies have their extra units in place to hit the war machines when we are forced to move out of our castle to flank and support the CC units.
Part of the reason for two canons is that one of them can be flaming, which is good against things like HPAs. However, the other one can't be, which is good in case they've got a 2+ ward vs fire.
I have never seen anyone take three canons. The closest I have seen is two great canons and a Stank.
As for using a wall - don't position 4" back. Position along the wall, as close as you can legally get. Poof. No problem. Impossible to hit you with a canon ball without first destroying the wall. 100% coverage, or there abouts. You can also use the silhouette of a house or other building, if you're sneaky about it. I don't know how your opponents behave, but when someone's trying to position their big model, I let them know when I can and cannot get LOS with my canons.
You do realize, however, that the odds of a Grudge Thrower getting a direct hit are almost 50% when the GT has the Rune of Accuracy, right? Do you also want to see that nerfed? It's equally dangerous to big gribblies *and* it can decimate infantry hordes.
Really, what *should* dwarfs have that's scary? Because we don't have a Magic phase, and our infantry is sub par when compared to other elite infantry units. People whine about Organ Guns, and they whine about canons, and they wine about stone throwers, but forget that we can't move and we can't cast and that the only other model type we can field is infantry.