Sure, it does offer scaling, but you really want 2x scaling - I tried non-integer values on a 5k display, just to get a sense for it, and it was as disappointing as expected. Slightly blurry text, rather obvious that this wasn‘t the recommended native setting.

In my opinion 2x scaling is the only option and that leaves you with an effective resolution of 1920x1080 points - sure, it‘s sharp, but far from true 5k and more importantly the same screen real estate we used to fit into 20-24 incch displays for years. It‘s the wrong resolution for 27“.

Yes, it does. Though at that point it's effectively rendering at 5k anyway, so any performance problems you have with a real 5k you'll also have with that setup.