Geoff ... You well know that adding weight to the putter shaft only increases inertia to the hands thus forcing the golfer to pull and push harder .. plain and simple.
As for the altered properties of the putter, it's a bit more complicated. Adding weight to the shaft will not only shift the putter's Center of Mass upwards, it will also raise and bring the putter's Center of Percussion closer to the shaft along the putter's longitudinal gravity axis (the 'hanging' axis going through the 'sweet spot').
Center of Percussion is only a function of mass distribution and if calculated for a putter or static golf club, it will be above the clubface sweet spot. (The only exception is if the clubhead 'disconnects' dynamically just before impact, which doesn't happen with putters.)
Even a small amount of 'backweighting' up the shaft will create a 'woody' feel to impact, not that it matters much for putters. Heavy-headed putter heads have been compensated with substantial weighting in the shaft at the grip end. Sort of counter-productive if you think about it.
Once again we have a pro golfer subjectively finding their own putter/putting solution ... and then marketing it to the ignorant masses .. but not you and me because we know better ...
