2) I'm not sure anyone should be taking advice from someone who installs NPC mods.
Xan is the only NPC mod I have installed. I just really like this character, sorry (I agree that most NPC mods are shit). In terms of strategic considerations, he won't change anything - you can exchange him for Edwin, who is even more powerful (slightly better stats, slightly better specialization) or Nalia. The only change that could be considered as a "cheat" is Minsca's transformation into a barbarian. But again - exchange it for Korgana and you get the same, only better.
BTW:
I just traded Jan for Imoen. What a pain... Imoen is behind the rest of the teammates by 1 million xp. Besides, it does not offer Detect Ilussion skill, unlike Jan. It hurts the most, even more than I thought. And she won't have a thieving HLAs.
Even if we assume Imoen doesn't have a xp lag, I still think that Jan is stronger and more useful. Yes - Imoen can be the team's main mage. But as an auxiliary mage, Jan does better. It is better to have: mage + thief/mage (multi) in your team than: mage + thief-->mage (dual). The mere Detect Illusion skill is invaluable (unless we have an inquisitor). There are also traps (regular and HLAs) - I personally hardly use them, but it's good to have them at our disposal in case of a really overcomplicated fight.
It's a pity. My conscience would not let me leave poor Imoen alone. Farewell brave Jan, hero of turnips! Hello, retarded sister!
What about giving up magic missle and fireball, is the +2 worth it?
Enchanter is ok - even strong - as long as we have a second mage. Enchatment spells do nothing if a saving throw is successful. So each hinder of the saving throw is very valuable.
Besides, it all depends on the playstyle. Some people beat the game mainly with offensive spells. I use them very rarely. Most of the damage is done by warriors; mages and priests are mainly for buffs, debuffs, summons and crowd control.