My B&W speakers

I would certainly not be shocked by that figure. I wouldn't call it a rule, but something to understand that an upgrade from a bookshelf that cost a grand compared to one that costs five does not bring five times the gain in relative performance. I would not be surprised that the inexpensive unit would put around 80% of the performance of the more expensive model. Putting a percentage figure on the whole thing is another matter and you can certainly always have a disparity in true value of a product in comparison to market competitors. In loudspeakers, I would expect to see greater gains. In cables, my expectations are quite low honestly. The 600 series is priced at around the third of the CM line. I would put the gains of the CM line at nearly a third overall. Then again, its only my opinion.

If it did happen, either the previous product was a poor or the new product is quite the accomplishment. The one thing is that performance gains is often subjective and system dependent so it will vary from user to user. In my own last speaker upgrade, the replacement had a retail cost increase of three times the predecessor. I would say that the overall improvement was only a little bit better then then a third overall. I was able to purchase the new speakers at a greater discount then the original so in the end it bested by a bit. My amp upgrades on the other hand are were not as great of a leap in comparison. Between two properly made cables, this figure can often be staggering in a bad way. You could spend ten times more and not even gain at all. If you did, I would suspect that something was out of place to begin with.

Inverse means opposite, so inverse relationship between cost and performance means that as you spend more performance goes down or gets worse. I think you mean "diminishing returns" which means the incremental benefit is not as great as you climb the investment ladder.

Increments of gain versus multiple factor. So I'm considering an additive level and not using the value as a multiple. Your take .33 and multiplying as compared to additional to make 1.33 instead.

But in general the concept you state is correct. If you would take the 600 series to the CM and compare similarly sized models, you would find they are close the three times the cost. It would not be outlandish to say the increment of a 1/3 gain or 1.33x improvement in performance between them. If you also to the CM to 800D series of similar size, its again not to far off this mark. The 800D series do have models that are far greater in size to achieve certain goals that were never even approached on the lower lines. Its part of the business model to provide something for everyone. I don't use the Diamond series as its a recent refresh and the current 600 and CM series are still waiting for their next.

You can see this business model being used to different effect in Kef, Focal, Monitor Audio, Dali, Dynaudio, and others. You would also note those who walk to their own drummer and have different ideas about speaker lines. Wilson would be kind of big, bigger, biggest, and bigger then biggest. But they do plan in this manner for their lineups and in consideration of what their competition is doing.