Francois:
I know that you know that I know that you know the tension of every string. So here's the question:
One old trick is to put on Spirocore solo strings but bring them up only to E,A,D,G. If they're never stretched to F#BEA, they don't get flabby at EADG. But...
What's the tension at EADG? Has anyone ever measured it?

:o

Don: I don't know! :D

However it must be lower then the Weichs, which is published by Thomastik, and also available on Quinn Violins.
Since the Weichs seem to be around 10% lower tension than the Mediums, I'd expect about 15% for the Solo, maybe 20%.

But tension is not the only factor to consider for the tone quality.
A thinner string also has less mass, and also a different elasticity and flexibility.
I've seen, for the same brand, thick strings with a clearer tone, thin strings with a darker tone (Helicore Arco).
It was the reverse with Flexocors.

I have a solo-tuning Spirocore G1 (A1) and it definitely sounds clearer than an orchestra-tuning one, both at orchestra pitch.

Hope this helps!