The manufacturer tells me they have Regular,...
The manufacturer tells me they have Regular, Medium and Wide fits. What are the fits sold by backcountry.com?
Actually, I got my answer. The Insulated Fit is the WIDE version while the Performance Fit is the NORMAL version.
I found that mentioned on the manufacturer's page but from the sound of it... the fit is actually identical. Insulation Fit on the wide last = Performance Fit on the medium last. I haven't not tried one on to know for sure.
Insulated fit boots from Salewa are rated for a EE width. Performance fit boots are rated for a D (standard) width.