My exploration of Sanchin is the result of decdes of working the kata as you describe and a quater century of Yang Tai Chi Chaun.
From what I've read, when Hiagonna Sensei originally taught Sanchin it was done a full speed and apparently Miyagi Sensei was involved in its 'evolution' into the form you describe.
When I got the chance to experience Ueichi Sanchin (also done a normal speed with normal breathing), I couldn't believe how focused my energies became in practice.
It is only when after long years of waiting, that I began to do my Isshinryu Sanchin (essentially Miyagi's version) full speed did I find the same energy release as the Ueichi. And felt the full force of its power.
Hence my study of the application of Sanchin.
As for the other values you mention, I'm dubious. The more I reflect, I don't see that tension does more than promote immobility, which is not where I see my art going.
The interesting thing is many attribute similar opinions of your Sanchin training to Tai Chi, too. Such as you're building Chi and doing moving meditation. But in reality you're not meditating if you're doing it correctly. Instead you're focusing on execution as in any martial art. And BTW, Tai Chi is meant to be studied (eventually) at faster and faster speeds, too.
The full value of Sanchin (either fast or slow) likely needs more study.
I'm not too concerned about value, I just continue to get the best rush I've experienced when I cut my Sanchin loose, and the application potential is where it's all about IMVHO.