I did spend a long time researching this a short while back to get to the bottom of what's Genuine and also what's best. I've also just completed not only 'bench' tests but in service use.
The ones from Mini Spares (which are made by Valley Rubber in the UK) are Genuine Cones that have been fitted to Minis since about 1976 up to 2000. The ones from W&P appear to be the same ones. The term 'Genuine' that I use here is in regards to how the Cones perform and last.
Those from P&L Minis are made by Avon, who mearly bought the Moulton Patent hence why the claim of 'Genuine' on these.
The Valley Rubber ones I found on the 'bench' to be as they say in regards to Spring Rate - they are genuine. Also, in Service, I found them to be really good and lasted reasonable well (given what I do to them!), also when warm (yes, they do warm up in use), they don't seem to loose their characteristics.
The Avon ones on the bench I found stiffer, however in use, they started off a tad stiffer, but soon got soft. I only got just over 5000 km from them and they are knackered. After removing them, I found the rubber compound had a greasy feel to them and some of the compound had 'transferred' to the flange of the trumpets. I'll test them again shortly on the bench to see what their rate is now. I don't know if it was the compound they use or if these ones weren't made properly however I have noted a friend's car, who fitted these about a year before I did has sagged a fair bit though he's done a few more miles on them than I did.
Needless to say, I've just bought a load of them from Mini Spares and I'll be going back to these.