> Does it make sense to increase the proof with high proof GNS? Maybe.
> The higher proof spirit will be more effective as a solvent for dissolving
> barrel "goodies,"
Well, that much is true, but it's not the whole story!
The hydrolysis of hemicellulose and other things in wood is accelerated by
increasing water content, so if your proof is really high, then you have
the ability to solubilize the goodies, you're just not producing them!
Piggott's book mentions a bourbon study showing that production
of color, volatile acids, and tannins all decrease as you increase
the proof at which you age.
So it's a trade-off. I think roughly 60% (120 proof) comes out to
be fairly optimal all things considered... but it really depends
on what you're after! If you're starting with pre-aged bourbon and
you just want mellowing, not extra sweetness and tannins, then higher
proof might be your friend.
Oh, and while I'm at it: smaller casks will give you more loss per year
than large casks. So the 5% figure might not hold. I have some numbers
somewhere comparing cask volume to percentage loss in Scotland... you
can triple your annual losses by using smaller casks. And by "smaller",
they mean the ~250 L (60 - 80 gallon) hogsheads (vs. the butts that
hold twice as much).
Prepare to pay the angels a fair bit if you're using ~10 gallon casks!