One can never argue with a listening impression. And I'm not saying you are wrong either.I would expect it to go up. These parts are very high precision and made of costly materials.
I agree that the horn is extremely important. That said, having compared several drivers on the same horn, the drivers can also make a large difference. Also, you need to match your horn and driver. Most 2" exit drivers are based on the original Western Electric design. These are drivers like the JBL 375 and up through the TAD TD-4001. These drivers have an exit that have a slow expansion and require horns with the appropriate throat geometry. Many of the newer drivers have more rapid expansion rates and require horns designed for these drivers.
Regarding minuscule differences, I have compared the JBL 476Be (original version, not the Radian sourced version) with the TAD TD-4003. I measured both drivers on the same TH-4003 horn and they measured similarly, but the TAD had slightly lower distortion, was slightly more linear, and had a slightly higher frequency mass breakpoint. These drivers are fairly similar, but the type of Be is different and the surrounds and phase plug designs are different.
The current US importer isn't in the audiophile business. Their clients are building custom studio monitors and other pro applications. This market is less prone to the marketing voodoo of domestic audiophiledom.
The Radian 951BePB measured slightly better than the JBL 476Be in the time domain in a comparison. However, the horn (which you also describe) will play a major role in such a comparison. The Radian 951 uses a 5-slit phase plug. If I'm not mistaken, so does the TAD TH-4003. I have no doubt that the TAD TH-4003 is an excellent driver and we could always customize our horn to work with the TH-4003 for those customers who would be willing to pay for it. But if it's worth it, is another matter.