I'm curious; how is Irenicus totally implausible?
He's an elf stripped of elvenhood which already feels constructed and definitely not epic. He doesn't look realistic and it doesn't make sense that this crippled elf is to be powerful, made with so much cheese. It doesn't make sense that his laboratory is directly below the center of a big city.
His actions don't make sense and he has suffers from the recurring villain who has to be defeated again and again syndrome which fits better to kids cartoons.
It's not reasonable how he kicks ass in the video sequences, it's only cheesy.
The 'recurring villain' bit isn't inherently bad; when used correctly (as I'd argue that it is in BG2), it can be a good way to give a sense of progression to the characters. Your first meeting with him has him effortlessly subduing you and your party, the second battle is winnable if you get some extra help, and the final confrontation is fairly easy for you, even with his 'upgrades'. This doesn't diminish him as a villain, it only shows how much you and your party have improved over the course of the game.
As for the rest of your argument, I'd say we'll simply have to agree to disagree, since it's pretty much pure opinion. (Not to say that the recurring antagonist isn't, of course.)