TST box is supposedly running about 150HP and 500Lbs of Torque at roughly $850. Propane kits are selling at $1,000+ and you aren't going to get 150HP without a blown hedgasket, and you definatly aren't going to get 500TQ.
You might get some power, and you will get MPG, but, the risk that comes along with it is usually not very justifiable. Propane advances the timing by acting as a catylist, and burning the fuel sooner. This makes cylinder pressures raise up pretty darn high. If you add more fuel, this advances the timing more.
Plenty of Headgaskets have been kissed goodbye by extra fuel + Propane.
Might as well get a KwikSpool turbo, and a TST Box. It'll cost a little more than propena, but power will be more, and it wil always be there as log you have diesel in the tank.
Merrick