I don't think it is the powder that is in question here, if your at the limit of twist rate to bullet length, shoot a lighter/shorter bullet or get a faster twist barrel......pretty simple.
Do the same with an IMR Powder and see which load hits instability first.
Agreed, and I like Varget. I am just pointing out a single experience I had that indicated to me at least that Varget is not temp stable in all conditions.
On the flip side I shoot a fair bit of BLC2 year round and never experience the fluctuations in performance that, according to some, its capable of. So yes as I think you are suggesting, there's more to the performace of a load than just the powder.