Base is 9% +tht so not even close (to SOs) when fully slotted.
CoX 50s: <ill/rad> <ice/ice> <fire/kin> <grav/sonic> <ice/storm> <earth/kin> <kin/elec> <cold/psy> <thugs/dark> <fire/dark> <dark/elec> <night widow> <EM/ninj> <mind/icy>
According to Mid's Hero Designer 3-slotting with level 50 IOs would give 14.3%, but you are suffering with ED by that point - 2-slotting would give 13.5% to hit.
Don't forget though that to hit is often more desirable than accuracy because it helps overcome high defense (I will not go into the maths here, but there are plenty of posts on the subject if you are really interested). I'd say slot the attacks with reasonable accuracy and Bright Nova with two level 50 to hit IOs.
As a matter of interest, Tactics (from the Leadership pool) gives the same base 9% to hit bonus (although obviously for the whole team not just yourself).
I can put in the maths part for why ToHit > Acc, so here comes.
The formula to calculate a NPC/PC's chance to hit against their target is this:
(Base ToHit + ToHit buff - Defence - ToHit debuff) * (1 + Acc modifiers) = X, capped at 5% or 95%
Let's try it like this: you have 13.5% ToHit, 1 acc SO of equal level in each attack and you're attacking an equal level critter with no ToHit debuffs or additional Defence the formula looks like this:
(75% + 13.5% - 0% - 0%) * (1 + 0,33) = 117.705% chance to hit, capped to 95% chance to hit.
If you only had 1 acc SO and no ToHit buffs:
(75% + 0% - 0% - 0%) * (1 + 0,33) = 99,75% chance to hit, again capped at 95%
If you had no acc SOs and a constant 13.5% ToHit boost:
(75% + 13.5% - 0% - 0%) * (1 + 0) = 88.5% chance to hit, again capped at 95% chance to hit.
Here we can see that ToHit > acc, but the +ToHit from nova form isnt quite as good as 1 SO. But remember, this (ToHit > Acc) is in most cases.. Although I cant think of a case where equal amount of acc would surpass ToHit.
Hope you understood what I was trying to point out.. 
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
My Katana/Inv Guide
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. -Einstein
So you're saying I should save some slots by not slotting my Nova attacks with ACC, and rather put another ToHit in Nova itself (already have 2)?
No I believe the best thing would be to keep the two in Nova and just one slot all the attacks with acc.
Main Hero: Mazey - level 50 + 1 fire/fire/fire blaster.
Main Villain: Chained Bot - level 50 + 1 Robot/FF Mastermind.
BattleEngine - "And the prize for the most level headed response ever goes to Mazey"
What I'm trying to say, that the 3 ToHit slots in nova do less than 1 acc in each attack 
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
My Katana/Inv Guide
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. -Einstein
Don't forget the attacks do a 7% defense debuff so you might get away with slotting acc in just one of the AoE (the one you open with). Plus the above figures are assuming SO enh - it's much closer at TO and DO levels I believe.
I've just started my PB, and I was wondering whether Nova 3 slotted for ToHit buff is as good as slotting one acc in all of the individual attacks?