The truth is that the 4200 Coupe Skyhook modes are only quite subtly different.
My car was without a rear ARB at all when i bought it. In that state, I wouldnt dare to negotiate urban streets without sport mode engaged, as it would readily bottom out the car- and given the very generous ride height, it was unexpected; so too the change once the ARBs was fitted. With uprated ARBs ( FD units) front and rear, the difference between modes became quite subtle, I even swore it had stopped working entirely...
Its clear then that there are many factors working in conjunction, and either with or against each other, to affect your ride. As you have reiterated, you ar no stranger to the geometry of suspension set-up.
All this to say you are reporting a ride characteristic we readily associate with the GS and the increased wheel rim sizing with the updated ECU algorithm. Perhaps you need a professional inspection to exclude the vagaries of internet diagnosis and anecdotal experience...? I suggest this because it would seem there is perhaps something going on, not usually associated with 4200 Skyhook
Your choices for remedy arent cheap, or necessarily straight forward. The essence of the issue is to first disconnect your suspension ECU. This is a simple thing, its in the boot on the left hand side over your CD changer.....If this shows you no dash fault illumination, you can spend on passive telescopes and set-up.
Ive seen plenty of cars being hawked for little more than £8-9K recently. This would inform most normal people's decision making..*
* There are folk amongst us, able to assist you in your man maths calculations, and guide you through the process of emptying your wallet quicker than you can feel pain- apply within !