Aramezzet

To illustrate the effect of mana regeneration on DPS, I'll show the comparison of cost between casting Frostbolts and Arcane Blasts, the difference in damage and how an increase in active Mp5 benefits the caster. I'm sorry if my theorycrafting is off. I'm out of practice.

Mana spent over 2.5 seconds of casting Frostbolt: 272
Mana spent over 2.5 seconds of casting Arcane Blast: 672 * 1.4 = 940.8

At 1200 bonus damage and 30% chance to crit:
Average damage dealt by Frostbolt : 2222.03
Average damage dealt by Arc.Blast : 2357.67

Average damage over 2.5 seconds:

Frostbolt: 2222.03
Arc.Blast: 3929.45

Difference in mana cost: 940.8-272 = 568.8
Difference in damage: 3929.45-2222.03 = 2707.42
Damage in damage, per mana over 2.5 seconds: 4.76

So each point of mp5 while casting increases DPS by about 0.952

While casting fully stacked AB's, the Pendant will be giving you 21 mp5 per 1.5 seconds... so that's 2.8 mana per second per second. I think an integration is the best way to find the effect of this trinket because of 2.4's changes of how changes to how the game handles changing mana regen are applied. Over 20 seconds, presented rather sloppily:

M = ∫(∫xdt)dt

M = total mana regenerated
x = mana regen per second per stack gain = 4.2
t = [0,20]
However, Arcane Blast takes 1.5 seconds to cast. To adjust to this I'll reduce time span t and increase x.
t-> t/1.5 = 12.66
x-> 1.5x = 6.3

∫(∫xdt)dt
∫(∫6.3dt)dt

M = [0,12.66]∫([0,12.66]∫6.3dt) //Yes my notation is shit but I hope you get the idea.
M = 1009.701
That's about 42 mp5. According to my previous theorycrafting that could be 40 DPS (though at your gear level I'd think it's only 30-ish. I dunno). That's huge. Go ahead and try it out.

Jesus Christ, I think I'll try to get this trinket and try it myself. It gets that much better with Icy Veins and the Skull to make AB's faster and stack the trinket's buff faster.