If you pull a lot of stuff, especially heavy stuff, often, then maybe the upgrade may be worth your while, although I would say that any upgraded trans you want to put in should probably be rebuilt as well. If you do not tow much, maybe you would not need to upgrade.
If you are going to rebuild a transmission, why not just rebuild what you have.
The best thing you could do would be to find a specialty transmission shop that knows what they are doing. Even if you have to drive a ways to get there, I would think it would be worth it. Most general mechanics today will not put the time and effort into a proper rebuild, and you end up right back where you started?
What is your mechanical expertise level? Would you be able to drop the trans yourself to take it somewhere?