I'm not sure you'll see any /increase/ in life, probagly a /decrease/. The 7.3 controller isn't bad, what with using the glow plug resistance as feedback for temps.
What it will do is keep the truck starting better with a couple of dead plugs... though remember, as soon as a plug dies, you have to use a little less plug time(though more than the controller would give you).
Personally, if the controller is working for you, just keep a few plugs in the truck and replace any bad ones as soon as the controller starts "short-cycling", where the time ON is less than the time OFF(it's easy to hear if you listen).
Once it does that, take a piece of wire and hook one end to the battery + terminal. Touch the other end to each glow plug's terminal in turn(wire unplugged). If it sparks, it's generally good. No spark = bad. Just replace the bad 1 or 2 and call it good.
