Well just finished an epic journey beginning at Candlekeep early last week and ending on the Abyssal Plane about 45 minutes ago. I used David Gaider's "Ascension" mod which spruced up the end of ToB quite a lot. As well as harder boss fights through the expansion, the final fight itself was bordering on unreal.
Other changes with the mod installed...there's some more dialogue with Imoen and she starts developing Bhaal powers too, culminating in transforming into the slayer...at a very inopportune time. Further, I didn't have to defeat Balthazaar, instead convincing him to join my party against Amelyssan.
So the last fight...it begins with Melissan summoning old friends Irenicus and Bodhi. Imoen also transforms into the Slayer making it a hard fight indeed....harder in fact than the last Irenicus fight. I kept Mazing Imoen/Slayer, and eventually made it through Irenicus killing him for the third time (I think, it may be more, I lose count) with only 2 reloads (one because I killed Imoen/Slayer and couldn't res her after). Then the usual "3 pools of Bhaal Essence" bit, only the fights are significantly harder. Still I managed them with no reloads.
Then the last fight. Fully buffed, armed to the teeth with the best weapons and armour in Faerun, with Balthazaar helping me, I had to reload about 8-9 times. One CTD, the rest getting my Immortal Arse handed to me. You see the new end fight has Amelyssan summon all of The Five (minus Balthazaar of course. Heaven knows how I would have managed a) without him and b) further, fighting against him). That's right, the invulnerable fire giant, the half-dragon, Sendai the Drow mage and Illasera.
Far and away, Sendai is the most aggravating and disruptive. No matter how many times I cast True Seeing, she was always invisible/santuaried. In the end, I won with a little cheese, I must say. My PC used Simulacrum and had the copy use all his bhaal powers, summons, dispels, etc. I went after Abazigal first, using Whirlwind attacks and took him down fast. Ilasera next after a poor attempt to Time Stop and deal with Sendai.
As soon as Ilasera went down, Amelyssan joined in. So we went to town on Yaga-Shura. I don't know what killed him in the end. I tried using "Harm" and whirlwind attacks, eventually something gave though I don't think I could do it again, whatever it was. Sendai next, a couple of breaches, ruby rays, no idea if they helped. All the while having to keep heals going with Rods of Resurrection and potions. Finally I got a break and the drow b***h went down by some miracle, leaving just Amelyssan.
Again, Simalacrum and summons, whirlwinds and healing as best as I could. Imoen was close to useless as her stint as the slayer (or a level drain I didn't notice) made her forget most of her spells. Getting Amelyssan from Uninjured to Barely and from Barely to Injured was insanely hard. It took maybe 5-10 minutes of pausing and micromanaging and breaching and healing just for that. This was a party that more or less walked over everything else in the game to this point. Once she got to injured however, it was easy.
After another minute of summoning, breaching, whirlwinds and what not, she went down and I vowed I would never install Ascension again. Further, there was zero chance I would repeat that fight to see the alternate (ascended) ending. All in, the failed attempts took about 2-2.5 hours and the one successful one was about 30 minutes, and I don't think I could repeat it again with ease...there was certainly an element of luck involved.
So yeah, Ascension mod certainly makes ToB....interesting. In the end not one party member died, Balthazaar made it and got a pretty good ending of his own. Jaheira (romanced), Minsc, Imoen, Viconia (romanced, alignment switched) and Sarevok (alignment switched) all had good and slightly longer endings.
But now, I can crack on with DE: Human Revolution! Phew!