Shaft Experts - UST AXIVCORE shaft in X Stiff - any thoughts?

As said by others, various Axivcore shafts available, and at X flex, could be one of the standard ones or an aftermarket or Tour van variant.

As a general rule, the 'real' Axivcores play true to flex, i.e. they are not particularly soft, and to be fair, so are the VS Protos, so I would concur with the view that going from the VS Proto in S to any Axiv in X, especially if it a bit heavier than the VS, is a distinct step up in stiffness.