I can't comment on how much pruning you need to do, but you certainly shouldn't prune plums in winter - all pruning should be done while the tree is fully active. The idea is that the longer the cut takes to heal the more chance of silverleaf getting in.
This makes grafting riskier in April, but many German grafting experts prefer to take early scions by breaking a small branch rather than cutting it. No tree would be here now if it couldn't survive blundering megafauna.
Everyone as ham-fisted as I am will have noticed that paper cuts hurt like hell but heal much faster than knife cuts.
Actually silverleaf is worst in plums but is also a risk for cherries, peaches, nectarines, apricots and almonds too.
Apples & pears are not completely immune but the problem isn't bad enough to stop winter pruning.
Cheers.
