Exhaust is the simplest and probably least expensive modification. After that, things get more complex and expensive. I'd go for an intercooler upgrade next, then possibly a different turbo. I don't know much about selecting the best size turbo, so I'd spend a lot of time researching before I went that route. "Cold air" intakes are probably not worth your time on this truck.

Finally, a propane injection system that was set to come on at a certain throttle opening or boost pressure would help with hills, passing and towing. Lots of howtos online and commercial kits that could be adapted. But, again, lots of research before hand.

I'll probably never do more than the exhaust upgrade.