I've got a full DTT rebuilt 47re trans with a 48re valve body, lots of money, but, lots and lots of fun, you'd be amazed at what an aftermarket trans will do for your truck. I also put on some traction bars on Saturday, what a difference. I thought my trans was the only thing slipping 6 months ago, those axles move the leaf springs around too, now I'm really putting the power to the pavement.
From what I've read, the 48re is a good trans until you start bombing your engine. After you make more horses you'll notice things slipping and the trans getting hotter. Mileage improvements will not be noticable because you'll be on the throttle more, and grinning! If you need to justify a new trans for yourself, just think more in terms a reliability.
Kevin