I know it's been said but...

[Mirage]

As some of you mentioned, the original purpose of arrows and bolts for thief was not to deal high damage but to be able to impair enemies with the additonal effects. This is the reason why a higher level set of bolts/arrows was not added for thief.

You don't need to bring a THF's ranged abilities up to the level of a ranger or a corsair. There's still a lot of room for improvements. If you want the purpose of ranged attacks by thieves to be to inflict status effects, then you should also consider giving thf a few new types of ammo. Silence bolts would be nice, for example, as would slightly more potent bloody bolts be. Perhaps also attack-lowering bolts would be nice to have, or even (gasp) dispel bolts!

In the case of dispel bolts, I realize you might think it is overpowered to be able to dispel almost non-stop. Perhaps the bolts should have a high delay, and the effect should perhaps not have the highest magic accuracy in the world. I do think that getting a dispel proc around 33% of the time would be all right, though. Firing bolts isn't free (and dispel bolts might be expensive to make too!), and doing several ranged attacks in a row lowers damage output significantly. Additionally, we wouldn't be able to use stat-boosting ammo such as raider's boomerang or other non-usable ammo pieces without losing all our TP whenever switching to a crossbow.