When you state that all modern Gillette blades are similarly excellent, I disagree with you based on my personal experience. Depending upon your beard and skin type and the blades you have evaluated, you may have that experience with the Gillette blades you have used. However, with my coarse beard and sensitive skin, I notice a significant difference in Gillette blades. However, I realize that I am not a typical shaver in that regard.
Astra SP blades are not quite sharp enough to give me a close, comfortable shave no matter which razor I use them in. Lest you say that I must have gotten counterfeit blades, I have tried SP blades from multiple sources; all provided similar, unsatisfactory results. I know that many people consider them to be plenty sharp, but they are not quite sharp enough for my beard. I could use them if I did not have anything better. I find the the Rubie Plus blades are not sharp enough, either.
I love Astra Superior Stainless blades, which I find to be both sharp and smooth.
Gillette 7 0' Clock Sharpedge Yellow blades are very smooth and are slightly sharper than the Astra SP blades, They give me a nice shave in an aggressive razor like the Muhle R41. However, they are not sharp enough to work well for me in milder razors.
I find Gillette PermaSharp Super, Gillette Nacet and Gillette 7 O'Clock Super Platinum Black blades to be very sharp, even sharper than that Astra SS blades. I find them to be similar to Feather blades in sharpness, although perhaps not on the first shave.. For me, these Gillette blades work quite well in mild-to-moderately aggressive razors, but they would be to sharp to use in my Muhle R41; the face feel would be too harsh.
Blades like Gillette Platinum, Gillette Silver Blue, Gillette Polsilver, 7 O'Clock Russian Greens, and Gillette branded Wilkinson Sword blades work fine for me in moderately-aggressive razors, but I prefer sharper blades like those listed above. I find these blades to be similar in sharpness to Rapira Platinum Lux and Voskhod. However, I can see why these blades are quite popular for many shavers whose beards are less coarse than mine.