what's the upper limit of various stats?

[Fiann of the Silver Hand]

Things like AC, weapon speed, and casting speed can all be modified up or down, but I can't find any resources that say if the engine imposes a hard limit on them. Use in-game for these is -20 to 20 for AC, and 0 to 10 for weapon & casting speed.

[Jarno Mikkola]

AC is limited to -10 at least to what comes to non armor modified states. Aka you can only have -10 AC from your armor, if the damage type modifiers are not taken into account. As for the weapon speed and casting times, I am pretty sure those two are the same timer, at least to what comes to the instant effects to be applied(without projectiles etc delayers) it's the round = (~6 seconds in 30 fps) divided by 10. And the instant...

[Kulyok]

If I remember correctly, normal BG2 allows you to have something like -26 AC, with multiple Blur spell.