I'm sure from what I've heard from people trying to fit wider tyres, and the other two P6's I've tried that on myself, that the variation in mudguard / D post dimension is common. But in a way I can accept that, because with 185 tyres it wasn't a critical dimension for Rover. Mind you, it must have caused fun and games on Denovo cars which were 205 section.
I've never heard of the problem with spring platform location in specific terms before. But then I very much doubt whether anyone has thought to measure it or had the technical knowledge to devise a technique before. I have heard of cars with springs of unequal lengths before, and to my mind that is a prime pointer to this problem. I'm also a bit suspicious of Rover having provided a procedure and the facility to pack springs with spacers. I've never heard of that on any other car before.
I've only actually driven four P6's sufficiently to form an opinion of their road behaviour. these were a '65 SC, '72 TC and '73 V8S, all when new, and Lucky of late. Of these Lucky steers and corners by far better than the first three, and of them the '65 was much better than either the TC or especially the V8S. All had manual steering. The V8S was like steering a ship! Besides steering quality the underlying stability of the car - its propensity to need steering correction - detiorated very markedly with the two newer cars.
Another factor that makes me suspicious was that Rover were not in control of base unit production. This was done at a separate facility by Pressed Steel Fisher. Such an arrangement is prone to problems in quality control.....
At the outset, Rover's vision was that the cars would not require "fitting" of the body panels - they'd just bolt straight on! We know today that this is a huge distance from the facts! I think it's telling that Rover had a specific gang on the line to sort the panel fitting out. That would have got up my nose if I'd been production director, so you can bet that Rover tried pretty hard to dispense with it.
So the answer to your question is that I don't have any specific evidence as to how common the spring platform problem is. But there are lots of pointers which suggest to me it could be very common. Basic engineering experience of what happens in real production situations tells me that, at the very least, there will be a batch of cars around my chassis no getting progressively worse until the problem was identified and corrected. My experience of the times, my knowledge of what was happening with Rover's financial situaution and the management upheaval inevitable with two take-overs in quick succession suggest that the identify and correct bit are unlikely to have happened. Again, Rover's intention was to have had P10 into production to replace P6 around '74. Inevitably that would lead to a lack of interest in what was happening on the P6 line. Especially with the demotivation resulting from a cancellation and start again instruction from on high.
So yes I do think this problem is very common.
"Lucky", '70 3500 (4.6 and ZF in waiting)
"Baby Rover", '89 216 VDP Auto 30k mls