The durability of the transmission really depends on how high you set the PPE programmer and how you drive the truck. I will say that I have not seen too many of the 6 speeds that limped, but when they are run hard, you will have burnt clutches when you take them apart. Given enough time and abuse, they'll start to feel it. If you want to add a little extra insurance, you can throw a Transgo Jr shift kit in there to help keep it alive. They can be found for under $100, and a competent mechanic should be able to install it in less than 2 hours even taking his time.
Again, the power depends on where you set the PPE and which one you get. There are four different levels to choose from. Their maximum power increases are 120, 225, 300, and 425. Those numbers are supposed to be gains at the flywheel, but if you get the standard Xcelerator (the 225 HP model) and set it on max, you will probably be in the 425 to 450 HP range at the rear wheels.
Yes, the Banks exhaust and intake are some of the best quality out there. They're just spendy.
