I appreciate the long and well-reasoned response. Is RAT and editing the hypercloud map image necessary to support the 1.25x size, or is editing the config and letting vanilla fill it with more systems as you implied sufficient?
Well, at least up to 0.95 Alex had left enough overhead in a few values to allow for just about a 20/25% increase in size or so, there's an old post by him buried in the forums somewhere where he mentioned that and indeed I could confirm that was the case in my experience. Basically you increase the size of the sector in the config, then start a new game and use the console command Survey All to see a counter of how many "markets" (planets/stations etc) and systems there are. Obviously the number is a bit random, but indeed you should notice the config allows for a bit of an increase in the markets and systems total amount (which didn't have enough free space to spawn at normal size) before consistently reaching the constellation cap (indeed around the 20/25% mark IIRC) and the sector beginning to generate noticeably less and less densely populated. I'd think it should still be pretty much the same today: there have been a few procgen changes since then, but nothing TOO fundamental.
That being said, we are talking about just an overhead and thus not everything might scale as intended (like the number of domain ships, sleepers etc that can spawn), so I'd say the best bet is still to use RAT to increase the themed spawners by 50% (a 50% increase is pretty much consistent with a 1.25x increase in both width and height, given that 1.25x1.25=1.56), so you are sure that EVERYTHING is set to scale as it should. If I'm not mistaken you can also turn off pretty much all other RAT content you might not like and keep only the procgen edit active, so nothing to lose there. That's very much hassle free.
As for the map (core/data/campaign/terrain/hyperspace_map.png), yes, if you increase the sector size you do have to edit it (regardless whether you also use RAT or not), or the clouds would only still fill a rectangle in the middle with the old size, which is pretty jarring as it's way too regularly geometrical of a shape. For instance, in my current campaign I have indeed increased sector width and height by 1.25x and the same I did with the map, which becomes 1012x625 pixels (vanilla is 810x500), so that the hyperspace clouds fit the borders perfectly.
Interesting factoid: seems it works pretty much the same regardless if you upscale the whole image by 25% (meaning the actual black shadow mask in the middle too), or just expand the canvas by 25% (so the black square in the middle stays the same size, and only the white background is enlarged at the borders). Depending on the software you use to upscale the image, the shadow map might become fuzzier, so the latter method might be actually better and simpler. But indeed there isn't that much of a difference, clouds are still generated and can change a lot depending on the density value you set (you can set it via RAT too) and if you have mods that add new fixed systems and constellations.
Imo this is a good balance to allow for quite a few content and faction mods without feeling cramped, and on the other hand without having silly distances to cover for missions or simply such a big sector that you'd never actually explore extensively before growing bored of the current campaign and feel like starting anew. And it's also still very performance friendly for my laptop.
On my desktop I did run much bigger upscales without issues too (I do tend to be one of those players for whom, instinctively, the bigger the better), but they just get rather unwieldly gameplay wise. Plus, honestly, I wouldn't anyway advise installing more than a dozen faction and heavy content mods (like Tahlan's, SWP etc) per playthrough at most, so there shouldn't be real need to go
that much bigger. It's not a matter of VRAM: it's because they are all nice and sparkly and they lure you in with their songs of pretty ships, rainbow lasers and totally out of place catgirls... and then you end up with literally
hundreds of different weapon types in your storage and
no goddamn way to sort them by manufacturer/design/name or whatever. That alone makes the game a pain as refitting ships can become torture after a while.
Anyway, I digress. TL;DR: yes you do have to upscale the map. You should be able to get somewhat similar results in vanilla without *needing* RAT if you keep below the 25% upscale mark, but RAT is just more foolproof and probably more comprehensive.