(Greater) Doppelgangers are important in BG1, especially toward main plot.
But, they are more powerful than in P&P in several ways.

I can't see any interesting things to do with them...
If anyone has suggestions, I'm listening.

Replacing them by another monster wouldn't be possible because of plot and quests involving them.

Either stick with P&P and make them less powerful. Either go ahead and make them better.
Last suggestion: improve them with P&P without lowering already higher stats and make their spells non-interruptable.

Just follow the PnP rules about Greater Doppleganger and too bad if they become less powerful.

Essentially, they will loose mirror images and haste, making them far less dangerous.
In Duchal Palace, it will be easy to protect dukes and kill them.

Additionnally, who would install a component that makes game easier?

Personally I care more that they follow the P&P rules than the difficulty of the encounter so I would install it.