Ya, the 70-xx Tweets each play a little lower and handle a touch more Power iirc. Of course, the tradeoff is with the 70mm ribbons, you get tighter vertical dispersion over the 64, and then the 70-20 will have narrowed horizontal dispersion compared to the xx-10s.
I don't know that I've really seen any measurements that show to what extent this happens in a well designed Speaker... and even if it takes it from ±80º to ±70º... Is that really a big deal?
I've always said I prefer a good 3-way, but if I ever were to buy a 2-way, it would be something Dennis had his hand in!
A different conversation with Dennis yielded his view that the Raal 70mm Tweets weren't really worth the cost compared to the OEM 64-10 (especially now that they have the 64-10x which lowers distortion). I think you go from something costing him around $80 each to the non-OEM Tweets that retail over ~$500 each!
IIRC, the 70-20xr crosses as low as 1800 with a LR4 filter. The 70-10 crosses around 2800!