I think for your situation you will have less trouble down the road literally if you just go to progressive front springs, because if you try to go heavier oil, more oil and or more oil and more air pressure you will blow the fork seals in short order. a litlle higher oil lever of 20w may work and no air pressure with the front tire off the ground, when you put weight back on the forks it will with such weight as you are describing allow for enough at full depression to hopefully not be too much to blow the seals out, just my view on it bro.
Wait, you mean for a GL1100 bike?
Sorry, my explanation may not work for those forks, I assumed it was for a GL1500.
I think for the 1100 it just needs higher air pressure.