I think I mentioned this in HIPS, but Kano described Shiro at age 16 as being dedicated, but not exceptional, and also described him as tiring easily, so that he could practice with him too hard or too long. There certainly was this legend that developed about Saigo, but it actually developed and flourished much later. In HIPS, I did my best to suggest that the idea that Saigo Shiro studied martial arts, much less aiki from Tonomo Saigo was extremely dubious.
Once again, I am pleased to note that my semi-interpretation, semi-intuition, is now caught up with research.
The following links, mostly by researcher John Zyl, who has access to the Kodokan archives, give a remarkable counter view to the entire history of the early Kodokan, of which Saigo was a part (which includes politics and aikido as well).
The second link has these two wonderful quotes:
But this is the period when Ueshiba rises to prominence. And I think he scared the sh!t out of the Kōdōkan.
If jūdō left behind the esoteric and the mystical aspects of Kitōryū (and ki is all over the place in Kitōryū and TJSYR) in favor of physics, if jūdō left behind the joint manipulation and striking in favor of a safe and healthy form of exercise … here was all of that stuff back from the dead, repackaged with the help of Oomotokyō. I've said this before but I think Ueshiba must have seemed like the ghost of jūjutsu past to Kanō -- everything he left behind risen from the grave.
Some kind of a meeting or special course in 1886 that gets transformed into a duel in 1916 and then into an epic battle that showcases jūdō's inherent superiority in the late 20s and 30s.
And here is supporting evidence from the "other" side - from Yoshin-ryu
What that suggests is the whole claim of Saigo Shiro learning Daito-ryu or whatever was yet another case of after the fact, "Oh, we do that too." (Note, too, that yama-arashi, his special incredible, must be from D-ryu technique is in all the early judo books, and I've seen films from the 1950's with a number of teachers demonstrating it - and it is sort of a midway technique between tai-otoshi and harai-goshi - - - -not anything resembling D-ryu.
the entire thread.