Hi, I had a similar question on the HUBB for a while with no answers. The standard spring rate is 90nm and as you know is way to soft for carrying alot of luggage. I tried to find a firm to tell me for a given weight what spring I'd need but the same as you even so-called specialists just said "we sell a kit" meaning a standard road kit. I also tried to find a spring to fit to the original shock but failed to. In the end I was lucky enough to pick up an ohlins secondhand and had it rebuilt and also put a 105nm spring on. Also on the front end I put Hagon "progressive" springs in (which are'nt progressive) to stiffen the front a little. I must carry about 150 kg including myself but not including fuel, but the way it's worked out after Russia, the Stans, Iran, Pakistan and India is quite good.
With luggage the bike is ridable on any roads and considering I have a 45 litre tank and have filled it on a couple of occasions on bad road this is'nt bad.
When I have no luggage however the whole thing is far to hard.