x2... there are no transmission failure problems.
There are only transmission failures due to user error. You can't expect it to live forever with a huge amount of added hp. And 100hp isn't necessarily a mild hp increase. You'll get close to the limits of several drivetrain components at that level. Nothing that can't be remidied, but a stock truck has limits.
If you want to run big tunes, and tow heavy with tunes, be prepared to dish out some cash. Build your tranny, and take the proper steps to ensure low EGT's, and then you can tow heavy with a big tune. It's not as easy as slapping on a tuner set on the max setting and just driving. Thats how you break stuff.
If you'd like I'd be happy to help you out with a powerful yet still safe package to get rid of your DPF, get you some better mileage, and tow better than stock, at a great price. There's plenty of options, but it all comes down to how much you want to spend.
Please let me know if I can help
-Jesse
Husker Diesel