I've done some testing with fireball in comparison with ice comet and chain lightning with the target dummies in town. First with single spawns, then with multiple spawns with fireball. Every time I used Harness Ether with it's 45 sec. cooldown. I cast each of the 3 spells 10 times each on the targeting dummie, then 10 times with fireball on 3 dummies. According to Fireball, it does the same damage mitigation to all 3 targets, unlike chain lightning which does full damage mitigation to the selected target, then decreased damage to all other targets.

With all my gear equipped, the only damage bonuses i have are ... Magic Damage +10, Magic Damage +2%, and All Magic Damage + 1% ... no elemental damage bonuses at all.
My int is 250 ... evocation 259 ... conjuration 233 (harness ether) ... crit chance 20.8 (%) ... crit damage 54.6% ... crippling hit chance 2.59%.

My Talents with max skill bonuses are as follows ... Fireball 23 (2517-3146 dmg) ... Harness Ether 5 (216% dmg) ... Meteor 22 (+15% fire dmg) ...Vizier Shielding 23 (should have no bearing on dmg paramaters) ... all other talents are at 0 ... I spent all talent points in the fire line and I'm currently level 58.

Here are the results of my testing;

...Ice Comet...FIREBALL...Chain Lightning.....Fireball cast on 3 targeted dummies
1.....5340........5172..............6693....................5535...2772...3077
2.....5094........4659...........10,560....................5721...2908...2916
3.....5568........5525..............5833....................5649...2910...2922
4.....5123........4939..............5833....................5141...2950...2970
5.....5392........5644..............8049....................4934...3912...2682
6 ....5208........5048..............9787....................7213...2843...2679
7.....8247........5364..............4543....................7846...2572...2767
8.....7832........5449..............4451....................8455...3130...3379
9.....5366........4825..............4991....................8807...2978...2964
10...5259........5747..............5664....................4768...2572...2663

now it seems to me, the calculation with harness ether just doesn't quite add up. I've also tested fireball without Harness Ether on 3 targets, and it seems to work fine.
Fireball, and maybe the other spells, should be doing much more damage with Harness Ether.

Maybe Harness Ether is not working correctly.

on the low end, fireball, with harness ether, should not be doing no less than 7953.72 dmg. (2517 + 216%) on all 3 targets ... thats alot of freakin low end damage, maybe harness ether dmg calc is wrong?

Sorry Maelfyn, i know its alot to take in, but the numbers didnt seem to add up. If you need more info, let me know.