It wasn't mods that oversimplified TNT cannons, it was the change to the dispenser in vanilla 1.5.
It used to be an engineering challenge to make a cannon that could reload and fire by itself...
As for modded TNT cannons, you're still generally stuck with vanilla TNT because none of the modders who introduced their own explosives thought to add the same "activates on dispensing" functionality to theirs.
BUT!
There are cool things you can do with mods though.
Make your cannon entirely self sufficient using magical crops to make the TNT and applied energistics to keep your cannon stocked with ammo.
Use MFFS or redpower (if you're an insane troglodyte who still lives in a post-apocalyptic 1.4.7 wasteland) to make the barrel movable, or computercraft to tweak the shot timers...all of these allow you to modulate the trajectory of the shot.
You can even use enhanced portals and put one infront of the cannon, then use a computer to specify a target portal, allowing you to bombard targets at will from other sides of the world or even other dimensions.
A bit of imagination and you can achieve anything