I have had mine for just over a month. I am running the 87 tune and have see about 1 mpg improvement. So based on mileage it would take a long time to see a return on the $300 investment. However, every time I drive the smile I get makes it worth the investment. I have an exhaust system in my truck and sounds better because it takes a little longer to shift. IMO well worth just for the sound lasting longer between shifts.
If you have the funds I would say do it because you won't regret it.
It shouldn't effect your longevity of your drive train, but that will depend on how you drive.
I cannot say that how much hp I have picked up but I can say that I do feel a difference.
Good Luck and welcome to the forumz and the Dodge Ram family. The best decision you have ever made in your life was buying a RAM.