Well if you look at the curves and grain profile of the Hawkeye color film, it is HD 400. The charts are identical. The granularity data on the charts indicates that a 4 point difference is "barely discernible" - and the data for Hawkeye/HD400 is within 2 points of Portra 400. Maybe it doesn't scan as well, but it certainly looks close to Portra in terms of actual grain. Of course this says nothing of color profile/contrast.
I decided to buy a little on ebay and see what transpires. I'll report back with a test in due course.