IMO, the differences between the 70-20XR and 70-10D are minimal after implementation in a good design. Two of the biggest differences are in dispersion and power handling. The 70-20 is wider and therefore behaves much more closely to a 3/4" dome in terms of dispersion IIRC. The other is in terms of power power handling and SPL. The 70-20XR is a little more robust as well, again IIRC. There are other differences, and certain specs can be specified by the designer (for the 70-20).
IOW, the 70-20XR is just as awesome as the 70-10 with a few small-ish differences due to the shape and size.
The Sierra 2 tweeter I just replaced looks considerably different from the 70-10D or 70-20XR pictures online. When compared to the 70-10D it appears the back box is absent (and the obvious round vs square face plate). When compared to the 70-20XR, it looks tiny, I mean miniscule. The box my tweeter shipped in looks smaller than the 70-20XR back box, haha!
If you said the Ascend Sierra 2 tweeter is interchangeable with the 70-10D (along with crossover tweaks) and be undistinguished, I'll run with it.
The 70-20XR, has much better thermal resilience, can be crossed over lower and does not have as wide dispersion, compared to 70-10D. The combined effect might not be insignificant. I've not heard them back to back in comparable design speakers, so this is an informed guess.
Last but not the least, there seem to be several versions 70-10 out there. DM mentioned the Sierra 2 tweeter cost $60 OEM (he acquired a pair of Sierra 2 with blown tweeters and played with installing RAAL replacements). I can tell you it was not much less than the Madisound price for a 70-10D (DF did give me a split price and I forfeit the damaged tweeter). I didn't mishear DM (now I'm second guessing myself, maybe he said $260) and I'm confident DF is not ringing in a 400% profit margin on the Sierra 2 tweeters.