This. It is a cardinal sin to assume that everyone who is effective is drawing from one wellspring. I feel that I have encountered multiple distinct sources of internal skills. Hell some of the sources I am working with have strategies so different from those described on Aikiweb that they do not even generate power. The people I have encountered with working skills are pulling from one or more of these sources, and undoubtedly from areas that I have yet to begin to penetrate. A lot of the people who are currently dabbling in these areas assume that the high level abilities are an individual's expression of a single skill, and worse, they assume that when they encounter someone who has working skills, those skills must be rooted in the same concepts they themselves are already practicing. Keeping one's mind open to the idea that you may not already be on the right track, that you may not even have a clue yet as to how someone's skills work, is the only way to create enough space in your practice to actually get traction in a new system. Of course time spent with anyone with working skills will help you get traction with other working systems, but please do not assume that they are definitely doing your stuff.
I agree with Benjamin (a certain sign of the Apocalypse!). At the moment a lot of people are in the position of being reasonably recently introduced to ki/kokyu/hara skills and until more expertise is developed there's going to be a tendency to think everything 'powerful', etc., is being done by the same means. A little reservation and wait-and-see is in order.
In terms of Ushiro Sensei, I watched him personally and I've watched some videos of him; I don't see anything outside of normal jin/kokyu skills (regardless of expertise, etc.). From what I've seen and felt of Systema experts, I haven't encountered those skills, as of yet. Personally, I tend to disregard a Systema - Aikido relationship, but of course each to his own.