Min-Maxing isn't exactly my area but, my impression was that the damage of the meta build wasn't that much different than other builds so much as it's a case where the damage is easier to accomplish.
Thus, the amount you'd need to nerf the meta build to have a bit more variety might be in the 5-20% range.
If the changes are spread across multiple abilities, the impact on actual non-meta builds would likely be rather limited because if you aren't on a meta build you aren't going to have tons of overlap with the meta.
ZOS could also do buffs at the same time to abilities from the classes in question that don't get much usage or that aren't helpful to optimized groups.