The average roll for exploding 10s' is 6.11. I know that from L5R.
That doesn't mean that Vis study becomes better in any way though, not even with stacking virtues for +5. It just becomes viable then & nothing more. Books, be it Summae or Tractati, are always better because they can be recycled, lend or sold & they don't endanger the Magi (botch die from studying & warping from higher auras).
My House rule isn't intended to replace them with Vis & go back the old ways, rather they seek to reinforce them so they become a viable option & as a secondary result to bring back some attention to INT, which I consider to be the most important ability of a proper Magus.
@loke, some context then. My preferred power level is slightly lower than what the Core rules allow, more in line with the major published NPC's, but with the acceptance, of course, that player characters will typically be the exceptions, those that go the extra mile in order to obtain the means to finish the story arcs.
I do enjoy higher power levels though, I remember reading the covenat_150 submission (a decade ago) & drool all over it but I have the ability to just ignore the power player inside me when I play or GM & go for more organic, more "realistic" & interesting concepts.
As for Vis supply, I pretty much halve the ArM5 suggestions, like it was used to be in previous editions. Even then, the PCs typically, still have decent quantities (relatively speaking) after a few years of play.
@Christian_Andersen, thanks for the insight & differences between the editions. I agree, adding concentration was a bit too much. INT, however, doesn't take that big values, probably a +3 unless the Magus in question burns a few virtues or when he becomes an Archmagus with sufficient access to the stat increasing rituals. I still think that books are the way to go in general & I have no problem with that, in the contrary, I think this is appropriate for the setting. I just helped equal the ground a bit. My changes tend to be low-key in general & I try for them to make sense whenever that's possible.