For groups, to flatten the transform, ungroup the moved group, and
regroup it (keep the same selection) - it's done rather quickly with
keyboard shortcuts Ctrl+U, Ctrl+G. Watch out for transforms of parent
groups (if nested), though and transforms on parent layers (after
resizing the page or changing its orientation).
This works. I already had transforms set to 'optimize' in preferences.
Of course I have 60+ groups on an average map so I'll look to scripting this as an extension but you've pointed me in the right direction.