They did fit rubber bushes on the TS and they wore out quickly so they retro fitted spherical bushes under guarantee and new on later models.
If you fit powerflex.. pull the spring out, you will find you have full suspension travel all the way to the bumpstop, no more resistance than with spherical bushes.. and for that to happen the RWS must be working as dictated by the geometry.
Very true, however let me try and explain why I think the Powerflex solution is inferior to the original set up (I welcome anyone who will correct me, as by any means, I am no suspension expert)
If you have a spring pan off the car with the original bearings in, and you put a bolt through it, you can move it quite easily around. i.e it will move on more than one axis easily, so up and down and side to side (see my poor diagram
e.g to move the bolt in the second diagram to that position with a powerflex bush would require more force than a proper bearing )
When you Powerflex the arm, you restrict movement in one direction, purely because you no longer rely on a freely moving bearing, you now rely on being able to compress the relatively stiff (compared to a bearing) bush. Therefore the original movement of the rear suspension is now stifled by the addition of this hard to compress bush.
I can not believe to compress a Powerflex bush (when the pivot is no longer central to the bush) requires more force than to move a spherical bearing, hence my earlier assumption that Powerflexing will numb the design features of the rear suspension.
Yes, the earlier cars had rubber bushes (Which I assume are more pliable than Polyurethane replacements) however like you say they were replaced with spherical bearings as they wore out quickly, which is perhaps an indication to the loads placed on them on more than one axis. Look at the rear upper wishbones, they only effectively have one axis of movement (up down), yet they never wear out, despite them being cheap rubber bushes. The ones on my 10 year old 92K GTV were fine, yet the rest of the suspension was knackered.