The current control is for the radius of a square region, as the UI indicates.
Minimum and Maximum have strict mathematical definitions -- and that doesn't allow for antialiasing.
And while a circle shape could be approximated, but it would be much slower than the current filters.
On masks, you can do the things you are asking for using the Refine Edge tool's Contract/Expand control, which will allow these adjustments with more control than Maximum/Minimum. Although I'll grant you, with Preview enabled, the dialog is slow as molasses on high res images.
Yes, because the mask/selection adjustments are using an octagonal version of minimum/maximum, and it's not quite as fast.
A true circle would be worse.
an octagonal version of minimum/maximum
…wish I could say I understood that Chris, but thanks.
Of course if you are using some of the most powerful machines/graphics cards that are available, and previewing new tools such as Refine Edge and "Quick" Select, and still not seeing anything like real time display on 8 bit single channels, then questions about efficiency do inevitably arise.
What would be very useful in those filters is a toggle for when we select the wrong one...
But they are mostly superceded by refine selections, indeed.