Some update, again, for the records ... I passed the MKIV to the " controle technique" (French equivalent of MOT), and apart from the fact it passed it easily, it was the opportunity to tweak the suspension.
I've read before that older Spax shocks have 14 steps setting (28 for newer), and you have to turn clockwise to stiff the damping, and anti-clockwise to soften it (of course !)... BUT, there's no stop clockwise, so the only way to know where you are is to fully turn it anti-clockwise, then count each click clockwise.
At first try, the efficiency of my suspension was rated 86/80 RL/RR ( in %) (remember I already played with it last week, but blindly ) and 75/63 FL/FR.. After reset, and after 10 clicks on each damper, it was respectively rated 80/80 and 80/81. Quite a good result !
On test drive, the suspension is far better , and I Assume that a setting between 8/12 clicks is the good compromise... Maybe not as good as new dampers, but mines are only 11,500 miles and it now can wait...