My 2 cents...
The Barrie 1/2 size down works great for me & I have a low instep. Grant last boots are not the best fit for me. I find they have to much volume in the waist or instep
when I go TTS, sizing down 1/2 results in a cramped toe-box. I would assume the grant would work well with a higher instep.
Interesting, because Thromb said that the Barrie is great for people with a high instep. Thanks for the input :)