When it comes to line pressure in the tranny, think of it this way, do you want to carry 50 pounds or 100 pounds. You can do both, but you are going to wear yourself out faster carrying the heavier load. In the case of the tranny, using the higher line pressure to make things move faster. A shift kit does it slightly differently since it has larger holes in the valve body which allows more fluid at a lower pressure to enter the pistons, allowing the internals to move faster, but not get slammed with the high pressure fluid.
You can also think about it using a 16 ounce claw hammer and a 3 pound sledge hammer. The claw hammer will drive a nail into a 4x4, but it will take multiple hits. The sledge hammer can drive that nail into the 4x4 with a single hit. The question is, will the nail still be straight after the sledge hits it. Much more likely to be straight with a few hits from a claw hammer.
If you are looking for things to help get the load moving, I would start with a good exhaust upgrade (stick with either a single 3" exhaust with a 50 or 70 series muffler or a true dual 2.5" setup with a 50 or 70 series muffler). You can go with some of these monster exhaust setups and they are great for when you are trying to climb a hill at 4,000+ RPM, but you will pay for it every mile on flat land because your low end power is going to suffer. After that, the GOTTS mod will be a nice addition. After that, convert over to an e-fan setup. While the e-fan setup doesn't result in major power gains (only gain about 6 hp and 8 ft-lbs of torque), the nice thing is you gain the same thing at 500 RPM as you do at 5,000 RPM. You will notice the difference when trying to get things rolling.
I would also recommend the shift kit in the tranny if not so much to get any gains, but like was mentioned, it helps minimize heat build up and in a tranny, heat is the enemy. Every 10F rise in the fluid temp shortens the lift of the tranny by half. So, if you raise the temp by say 40F, you have cut the life of the tranny by 16 times. Getting a tranny fluid temp gauge may be a good thing for you to get so you can keep an eye on things. A short burst (a minute or two) up in temp is normally not a bad thing. IT is when you hold the temp up is when you start having issues.
Unforutnately, any sort of mods you do is not going to help the overall capacities of the truck. That is pretty much limited by the tranny and driveshaft. You can contemplate going up 1 step in gears (ie, if you have 3.55, look at 3.73, if you have 3.73, look at 4.10's). This will not hurt the mileage too bad (maybe loose 0.5 mpg or so), but the big thing will be learning to keep the RPMs to under 2,200 RPM when towing on level ground. Having played with my truck, you start keeping the engine RPMs up above 2,200, you will see your mileage really start to fall. The further from 2,200 you go, the quicker it approaches 1 mpg. Granted, plan on spending about $800-1000 an axle. So, if you have a 4x4 truck, this can get expensive in a heartbeat (ie, cost you $1600-2000).
