Sorcerers do not even notice gear other than stuff like the robe of Vecna, rings of Wizardry/Alacrity, etc. Once a sorcerer gets PfMW alone, his powers exceed that of any item in the game. As for horrid wilting, I don't recall it bypassing magic resistance, but I think you vastly overstate the level 3 fireball's usefulness; between the low damage cap, exclusively fire-based damage that many things are immune to, and the high magic resistance of many later enemies, I found it pretty difficult to get all that much usefulness out of it towards the mid-late game. Horrid wilting's casting time is a bit long, but it hits so much harder than fireball with an immensely more reliable type of damage that it more than makes up for it. It also doesn't hurt party members, so it's much easier to hit a large number of enemies without hurting your own guys.
Fireball is the AOE damage spell of choice in low level DnD, but once efreetis, demons, and rakshasas become commonplace enemies it begins to lose effectiveness.
As an aside, the other five *anything* become tagalongs in a party with a sorcerer in it. They are nigh-indestructible time traveling superhumans in a pre-industrial world. If the other five guys are sorcerers too, it's like having 5 boring invincible superheros and Galactus. They can all destroy planets, but Galactus does it faster so they all sit around contemplating their navels until he gets hungry and needs to re-memorize his cheesy powers. If they aren't sorcerers, then Galactus might as well be soloing most of the time. You might think I'm exaggerating, but I've gotten a lot of sorcerers through the game with 6 people of various different classes; they're just there to make the early game bearable.