what tyres are you running ?
I've tried most makes of tyre, and always run 42 rear.
I find anything less than 38 in the front is noticeable as I said.
If the rear is lower than around 38 it too is noticeable even when solo with no luggage. Its most noticeable when getting on the power early out of bends, or in long fast bends where there may be a bump. It feels like the rear rim is rolling in the tyre, funnily enough just like a flat tyre would do
The more you crank the bike over the more it feels likethe rear end sliding away.
By the way, that price for the drop link was a couple of years back now, so it would be a good idea to check first thing monday morning before you make a decision.
To be honest, I will happily ride a bike regardless of compression or rebound settings, but bad tyre pressures I really dislike. If they are wrong, it matters little what you do with the other settings as the bike won't go where you point it.
One final thought: the 4S has the adjustable preload knob, how much of this have you got wound out ?
Either measure the spring length with the bike on the stand, or how much of the adjuster ring is protruding.