The texel density is 2 (4?) per game unit (so, a texture scale of 2 in radiant). I'm using external lightmaps.
I would much appreciate it if anyone could shed some light on this

That's a bummer :/ I was getting pretty good results using _lightmapscale and external maps, aside from the blur.Its not possible to render textures perfectly 1:1
This has been my mode of thinking, but put it aside to try this out (which I really wish was viable).High resolution lightmaps might sound really cool, but in practice it's extremely hard to notice the effect, and you'll run into a bunch of limitations which usually means the lightmap will be downsampled to fit geometry, run out of memory, or other issues. Instead, I would rather opt for better quality lightmaps using a combination of entity keys, shader directives, and compile settings.