How much of a UPS improvement should I expect from this mod?

That depends on a lot of variables so doesn't have a straight answer. Probably not all that much overall.

Okay thanks. Put another way, would you have any confidence that there would be some performance improvement, even if you can't say anything about how big it would be? Would you have any confidence that performance would not be worse...?

For context, I'm looking for ways to optimize UPS in megabases, including mods, but without making radical balance changes. When I'm deep in the late game I don't need the remote scanning capability of radars: mainly I just want visibility so that I can place ghosts and mark things for deconstruction, and also spot problems while I'm riding around in spidertron on the other side of the planet.

I'm hoping that this mod would be a strict upgrade in these scenarios, since the remote scanning is completely useless to me, even saving 50ms on my update time would make the conversion worth it.

P.S. I noticed that you have a conversion-friendly recipe, which is a big help in that regard, so thank you very much for that.

I would expect the performance to be better, assuming you will use fewer regular radars. I don't really see how Vision Radars would make things worse; only better (even if maybe not much).
The main thing that will impact the performance with radars is uncovered chunks. Chunks in turn cost performance with potential enemies on them, as well as pollution calculations (which is also why disabling pollution altogether can make a noticeable difference; although that would of course change the gameplay if you play with biters).

Point being; if less chunks are uncovered because you use Vision Radars instead of regular Radars, and that leaves chunks not discovered (so also not by artillery shots, manual exploration etc either), then you should be able to see some performance improvement.

If you already have a save with the chunks uncovered, there's also a console command to delete chunks that don't have entities near them which also would help the performance, but of course also mean you no longer have a view of them either. I don't remember what it is, but i'm sure you can find it easily enough.

Gotcha. Good points on the effects of covered/uncovered tiles, pollution etc. I plan to set VisionRadar to use the default range and then replace all Radars in my base with VisionRadars, for no reason other than because when I'm in the late game I don't have any use for the active scanning that the stock radar does.