I play IWD 1 at the moment with a system like T.G.Maestro's The Dex penalty is oriented around the weight of the armor. The heaviest (Full Plate, 70 lbs) gives a penalty of 7. Armor between 51 and 60 gives -6 and so on down to 1-10 giving -1. Since magical armor usually weighs less than normal armor this reflects the enchantments too - at least a bit.

Shields lead to even more penalties (Bucklers, Small: -1, Medium: -2, Large: -3). Here I didn't take weight into account but maybe it could be done here too.
Resistance: I think your numbers are way too high. A cleric wearing a full plate (70 %) and casting armor of faith (max. 25 %) is nearly invulnerable (95 % resitance to all). I have split it up as follows:
Leather: no enchantment to +5 gives 7 to 12 %.
Chain/Splint: n/e to +5 gives 9 to 19 %.
Plate/Full plate: n/e to +5 gives 14 to 24 %.
Then I looked at the special AC bonuses (f.ex. plate mail: -3 vs. slashing) of each armor, applied that number as an additional percentage (normal plate mail now gets 17 % vs. slashing). With shields I did the same thing. (Bucklers/Small: 2-10 %, Medium: 4-12% and Large: 6-14 %, special AC bonus converted into % increase/decrease).
I know, this isn't realistic (f. ex. the DEX penalty doesn't take the character's strength into account) in any way but its quite playable and a bit more believable than the original system. And I actually tend to keep my magical armor. A chain mail +2 now is a better suit of armor than a normal plate mail. In vanilla BG2 or IWD I'd sell the chain mail, buy a normal plate mail (same AC but waaay cheaper) and a lot more stuff with the remaining gold.

Wolfgang