Personally unless you're a regular trackday yoof, I'd try them FULLY backed off to soft & the fronts too (the front can have a surprising effect on the rear end feel)
On the advice of CooperMan, I had the rear SSL adjusted, which has resulted in an improvement, although I still don't think it's a £4-5k improvement over the stock suspension.
I do now have a horrible squeak accompanying any lateral movement that wasn't there pre-SSL. Does anyone know what this might be?