I came up with a question that I do not know how to answer on my own.

Would one 64x64 pixel turf use less resources than placing 4 32x32 pixel turfs to cover the same area? Or would both methods use the same amount of resources?
The 4 tiles would use up more as they each have to run through their own obj in the list, but the 64x64 wouldnt have all the sides densified.
64x64 would be faster if you need only to draw it.
You'd always have the exact same amount of turfs anyway, and the performance difference between both implementations is probably negligible (well, with that small size it should definitely be).
I don't really see the point of not doing it normally (each turf having an icon which is shown and has the size of world.icon_size) unless the behavior of using the 'unconventional mapping' specifically benefits you in some way, just because it's more native to the system for an atom to have a single-tile icon. Really, I think using the native big icons functionality with turfs is, most of the time, abusing that feature -- as it's not needed for mapping.
Too bad the map editor does that automatically instead of automatically splitting the graphic between turfs for you like before (well, you'd get that old behavior back if you set world.map_format to TILED_ICON_MAP).