Edit: Thanks Njald, rookie mistake

. Fix'd.
Overall the destro changes are not much change, and make us more RNG (and increase our crit scaling).
Here is some napkin math, based on my breakdowns from the raid last night.
I had 65 Emp Imp, so that is up to 52 bonus crits with the changes. With a 46% crit rate thats 28 actual extra crits.
My cast breakdowns:
Spell | hits crits | % casts of total | # extra crits | extra crit damage
Incinerate | 197 173 | 54.4% | 15.2 | 107k
Conflag | 50 63 | 16.6% | 4.6 | 32k
Immo | 65 41 | 15.6% | 4.4 | 13k
Chaos Bolt | 57 34 | 13.4% | 3.8 | 30k
Total extra damage = 182k (before FnB nerf), ~190k after nerf
Loss due to FnB:
Total CB+Inc damage = 4843 k
nerfed damage ~= old * 1.1 / 1.15
Nerfed damage = 4632k
Loss = 211k
Overall loss in damage : 21k out of ~8.5 million, less than 0.25%.