I once tried this with a propanetank from a outdoor grill and one of those pressure regulators that comes with them. I found some torque gain in 4th and 5th but no significant change in the lower gears. I belive if you use an adjustable regulator that gives more flow than the fixed ones on camping apliances you can get more HP. I still have all the parts needed to since I never got around installing it in my pajero, it's a solenoid, flow limiter, a couple of nipples and a button at the price of 2 dominos pizzas.
I'll maybe throw in a pic of the parts today.