Just a quick post with a real-life experience for anyone reading this thread considering the 5 link. I had the front SSL kit fitted to my Plus 4, which made a big difference and was good value for money. The rear, however, which was fitted a month later, was a complete waste of money. It made practically no difference; if anything, the ride is slightly harder and I preferred the car as it was.
It may be that my (2019) car had a relatively decent ride quality anyway for some reason such that I didn't notice the difference, whereas an older car would benefit more the 5-link. I don't know, because mine is the only Morgan that I've ever driven. In any event, the rear 5-link is certainly not transformational, which it should be for the money.