I can understand that you like the PnP rules.
But this would make powerful classes even more powerful.
When I look at the NWN2 build maker, warlocks and sorcs are very popular classes, together with bards.
Only one build mentioned that it has a chance to beat a sorcerer (cleric, stormlord 10, dragon disciple10, becoming a storm elemental and being lv10 RDD gives immunity to some powerful sorc stuff)
Warlocks can cause AoE damage that causes additional effects every round without spending resources.
Sorcerers have many powerful spells and they can chose to use meta magic whenever and however they want.
Making them more powerful would ruin any remains of balance more, and game balance in DnD is already bad.
Playing a game at the highest difficulty is supposed to be hard.
Games are usually designed to be played with a full party at normal difficulty.
When you have problems on the highest difficulty (maybe even solo) than everything is fine.
If you had no problems, either your class is totally OP or the game design is really bad.
Forget the balance thing. Forcing balance down the throat might lead to boring gear as in PoE2, probably the worst gear I've seen in any RPG.
BG is a timeless classic because of the crazy shit you can do. Hell, is there anything more fun than finding bizarre combos with UAI?