the good the bad and a way
The good, yes you can "turn up the fuel" or add fuel and get more power.
the bad, When you start making smoke out the exhaust you also start running hotter exhaust temperatures. Get hot enough for long enough and you will damage engine parts.
Back in the day when these engines were new, turbo kits were sold to get more poop out of them with out killing them too quickly. By adding a turbo, with an intercooler, and turning up the fuel a little, you will get more poop, keep exhaust temps down, and a cleaner exhaust. The internet was not much of a thing at this point in time and most of the message boards that were active about this are now thirty years old, There should be books and some internet information about safely adding fuel, turbo, and intercooler. I would guess you could do as much as 100 hp and 150 ft lbs of torque and not put too much stress on the engine, but I dont really know for sure. I have an 89 uhaul with the 6.9 non turbo diesel and have looked into "turning up the fuel" but will probably install 95 or newer 7.3 instead and then add fuel/turbo/intercooler.