Afaik:
Fireball doesn't do aoe dmg if it pierces, fork or chain, on the target it does that. It only does an aoe on the final target hit(for example if you have 50% to pierce, shoot at one mob, it pierces the first and explode on the 2nd, first will get hit by single target dmg, 2nd mob will create an aoe). Concentratred work fine on the explosion. LMP/GMP can be used to shotgun mobs as expected.
Fireball is alright but not great. It does pretty good dmg if you LMP shotgun a mob in the face and with a quality fireball+flammability, it has a high chance to burn even on non crits, with proliferation you then don't really "need" fork or chain, since the burning is applied to everything around for equal dmg, it's just a bit slow since it's over time dmg(but then you get 70%burning from searing touch and like 60%more for the burning big node in the passives). Also the spell sucks dicks until you have a bunch of supports on it.
That's not entirely correct. If fireball pierces, it does aoe on the first target but not any subsequent targets--(how would the game determine last target until it was too late?).
I will be adding pierce as a support gem eventually because it will allow better penetration of burning into large packs.
LMP, chain, and fork would not be a good choices for this build because it reduces the damage of each fireball and therefore the damage of the burning that gets proliferated. Similarly, casting speed is not particularly useful for this build because burning doesn't exactly stack (mobs keep all the burning debuffs but only the highest damage one at any time causes any damage).
The spell is very weak for single targets.
I've been curious about how concentrated effects would work, but the aoe on fireball is small enough already that it doesn't seem like a good idea and I already have 5 support gems picked out (EP, chance to ignite, pierce, fire penetration, reduced mana).
This is my current build:
http://www.pathofexile.com/passive-s...QR5oHsGPF2_c8=
I'll be picking up the fire/lightning resist nodes to the left of start then going for pyromaniac, the life nodes to the left of that, a dex node for pierce, the 8% energy shield near the start that I skipped, then maybe immolation or more life/ES nodes after that.
I'll be getting doedre's damning for the +1 curse. I can talk about curses if people are curious.
Edit--Just looked up concentrated effects--it doesn't affect main target damage (supposedly)
http://www.pathofexile.com/forum/view-thread/84798