With the nelson tune mine is shifting at 5900rpm with the tow haul mode on or off and I do have the line pressure increased above 70% throttle.
With it on- the trans does seem to shift a little harder especially at part throttle, its hard to tell about full throttle.

I still have about 50% TM left and I would not say the shifts are very hard. I just feel it was shifting out quicker in tow haul mode.

I will do some more testing with it on and off.

 

after reading this post I decide to do some experimenting Ive got a 91-93 nelson tune on an 02 sierra 5.3 Tow mode I noticed it shifting around 5400-5500 wot where as with it off Im shifting around 400 rpm higher. tow mode I noticed more torque on take off from my accurate butt dyno with tow mode off 0 to 60 seemed to be quicker. So it seems the nelson tune on mine didnt do anything to tow mode I still get more TQ down low but less top end.

I cant wait the the 3000 stall shift kit and servos that are going in next week

 

Since I have the tach with the highest memory recall, I'm going to try the tow/haul on tonight and see what happens. I have a shift kit/vette servo and a nelson tune, but I don't think he increased the line pressure any. I'll check it out and post up the results.
I do know for a fact(track proven and street proven) that my old setup with the 4.8L and the superchips tuner ran faster times with the tow/haul on. Don't have a clue why.

 

OK, I ran the test using my cheesy G-Tech and here are the results:

With tow/haul on:
0-60 mph - 6.45 sec

With tow/haul off:
0-60 mph - 6.25 sec

There you have it. However, for just scooting around in town, tow/haul mode is a lot of fun!

 

Tow/haul no longer helps me out. In fact, it made my tranny do some weird things now. I will no longer be using that button, unless I find a way to use it to activate a bottle. That would be a pretty cool sleeper setup for nitrous.