Sutherland's says you should have at least 3 complete threads for the locknut to engage, if you can gain that just by some "heavy" facing, I'd go for it, but would not try anything but a real head tube mill (not a file) if you expect things to stay parallel and aligned. The DA already has smaller balls than typical (which is why its SH is lower than most) and I would not substitute even smaller balls since the radius (ai) of the races/cups is designed for the contact of those balls.

Re the below: the DA7600 is the shortest stack of all the Shimmy HSets, the Exage R500 then the Sante is next (good luck finding one), Deore M730 also short but not sure if that's 1" or 1.125"