when I had my plus 8 it also had very harsh front suspension, new tyres, AVOs and BRBs made a huge improvement to the point where it was manageable. On my current Plus 4 the front end is totally standard and much more comfortable and hence enjoyable. The harsh front of the Plus 8 was one of the factors in the decision to change.
Given that it's the same basic suspension design, I wonder what the actual difference is between the Plus 8, your Plus 4 and say a 3.7 Roadster.
Can it simply be the spring rates ? Am I right in thinking that the Plus 8 may have the rebound spring in compression and the others don't ?