EDIT:
Crit Hit seems to give you +10MAB when it procs (Thanks Suiram). Unknown if this affects QD or enfeebles.
I got Witch's Sash and Wise Strap today and started testing a bit on puddings first. Here's a quick summary of my observations...
Lightsday | 131 INT | 67% MAB (with trait included) | +15% stave damage
Note that puddings have a +25% damage taken to magic.
Thunder IV - Normal 1587 | Critical 1677
Blizzard IV - Normal 1526 | Critical 1612 (NOTE INT is now 136 from this b/c of +5 INT on Aquilo)
About a 5.6%-5.7% increase in damage. When doing puddings, the amount of damage on the "critical hit" was always the same and completely consistent. I wanted to check if this damage increase was a straight increase or if it changed something within the damage formula...
I tried testing this using low level spell spamming. I haven't been able to do much but this is what I have so far (mob tested was Sabretooth Tiger in Batallia)...
Lightsday | 90 INT | 32% MAB (with trait included) | +0% stave damage
Water I - Normal 81 | Critical 88
This resulted in a damage increase of 8.64 to 9.87%... a significantly higher amount than that tested on puddings. This leads me to believe that the critical hit damage increase is not a straight up % damage added on top of your normal damage, but involves a manipulation of 1 or multiple factors within the damage formula variables.
I know the data is limited at the moment, but I was wondering how the variables could possibly be manipulated in the formula. I came up with these factors (or some combo of them)...
1) Straight damage increase (like staves) - seems not true with this data
2) Increases the player's INT or alternatively decreases the mob's INT (same effect)
3) Increases the amount of MAB in the formula
4) Increase the BASE damage of the spell itself (for instance, Thunder 4 is 541 base)
How exactly does melee crit hit work? I know they aren't exactly comparable but maybe they have similarities?
Also, Elemental Seal doesn't guarantee you a critical hit. The proc rate does seem to be about 5% with both Wise Strap and Witch Sash. Overall it doesn't seem to be an overwhelming boost. I'm still debating whether to use Wise Strap at all. Witch Sash is the best nuking piece with or without that critical hit boost in my opinion though (because of easy sorc ring proc).