[Amergin]

just compare it to other items in the game.

base: studded leather +4. that's fairly powerful, on par with shadow armor minus the paltry stealth bonuses.

-3 to casting speed. the -4 of the robe of vecna is considered by most to be its most important attribute, and this is an expensive and unique item. this combined with the AC requires you to sacrifice some other useful items to make it, imo.

as far as "too powerful", it depends. are you soloing or using a full party? in either case you'd be better off with the robe of vecna anyway as it's only 1 AC higher with better abilities otherwise and it's already legitimately in the game.

on the other hand, you could just pretend that the robe of vecna doesn't exist and use this instead. i much prefer a weaker magic item for my main character if it fits his style rather than some superitem that doesn't (e.g. i don't care how great crom faeyr is, i'll never have my elf F/T dual wield with it; it makes no sense.)