There are some concerns for sure regarding UPS. I really like that you put effort into minimizing UPS impact; from a glance however, it's hard to see how much of an impact it'll have on a running base.

It's definitely a great mod for people that are satisfied with their map after they launch their first (few) rocket(s). People attempting 1 RPM or more may suffer from the UPS cost this mod has, but we won't know for sure until people manage to do some serious testing with this.

Either way, it's a cool idea and a feature a lot of us wanted. And I certainly don't want to discourage you from optimizing this even further if it's possible. Thanks!

I've put a lot of effort into UPS optimization. By default, each surface<->underground connection is only processed once every 5 seconds. I haven't done extensive testing with very large bases, but my expectation is that this will end up using considerably less UPS than, say, LTN, and I don't ever see concerns that LTN is too heavyweight for large bases.

There's also an in-game setting to change the update frequency, trading off more items "wasted" in buffers to allow for even less frequent processing, and thus lower UPS costs. As your base grows, you probably care progressively less about relatively small amounts of items sitting in buffers, and so you can reduce UPS cost as far as needed. I don't foresee any fundamental limits to this. You could have a connection that buffers minutes or hours of production, then transfers it all in a few API calls for almost zero UPS cost.

the utilization of what are essentially loaders and mini chests as well as a reduced update frequency should mean, that, so long as you don't exclusively use this as your sole method of undergrounds in your base, you should see minimal impact. IE: only use these if another solution doesn't work or is too difficult or messy. If these are used exclusively for all underground work, a late game base UPS will likely see some additional UPS issues because of all the "Extra" calculations the game is still doing with the loading/unloading and processing the item transfers between surfaces.

although, the UPS issue with belts is mostly a graphical one as individual entities have to be rendered on the belts. as for the vanilla undergrounds, they essentially act as linked-chests that have two entities a max distance of "n" spaces apart based on the belt level. the undergrounds require very little processing in terms of rendering compared to their normal belt counterparts. but, if the beltlayer were used for more...long-distance runs of items, that may, in fact IMPROVE UPS. by reducing the number of total belts the game has to render. there's still the update issue that each belt needs to have it's contents updated each tick, so...no getting around that until you switch to logistic bots. but, there are other benefits to using this mod. (like the ability to belt-weave in a fashion not possible before)