In theory, yes it will be better, I can't find exactly where I read this right now but a tbb is more accurate at shorter distances whilst a wider bore more accurate at longer distances. Given this and the fact that a pistol is typically used in closer situations it would improve the performance. Gbb pistols also have a relatively low fps and a tbb will increase this.
This discusses something similar http://forums.airsoftmechanics.com/index.php?topic=9749.0
Personally I'm not sold on them, the ranges I use my pistol don't warrant it and most people i have spoken to who have installed one haven't really noticed much of a difference