Given that entry line and enthusiast AMD gfx cards scale in their mhash per Watt, I see zero argument / sense in differentiating between "hobby" miners vs "pros".

How could there be? The MARGIN stays the same issue regardless of how much VOLUME you have.

If it somehow becomes unprofitable for the current AMD generation of gfx cards, it becomes unprofitable for _ALL_ of them.

Regardless of whether it is some dude running 10 machines of 6990s or one normal guy with a 5770 - setting aside other hardware components / power draw sources, of course.