Ice pack you are spot on,
It's what I was holding in my head but didn't have time to search so didn't want to quote! I too heard all sorts of reasons for it but could never find a definitive answer so I sort of shelved them as I never got to the bottom of it. But owning a plus 4 I did hold onto the fact, and it is fact, that the rebound spring is held under pressure on that model and I am expecting to be shot down re. the rest, although reaching deep into the memory it was something like this....... The 4-4 was a gentle A and B road pleasure car and the Roadster being considered a tourer for crunching the miles had similar demands.....so a similarly balanced set up... WHAT, I remember thinking, A V6 rocket, that doesn't make sense. And now here's the bit I really didn't get. The plus 4 was always considered the more powerful car (might have been sportier)so is expected to be pushed and the set up here reflects that!
Well this would be a nice one to put to bed, hope someone knows the truth here as I'd hate to think we are creating a myth to join so many others.
