The actual actuation speed and power of the RD shift motor are functionally identical. Absolutely no reason to change a RD7150 for a RD8150

I can say as the owner of bikes with R8100, R7100 and indeed R9200 that the front mechs on R7100 are not the same (R8100 and R9200 essentially are). R7100 is the older design, same as R9100 or R8000 and larger, heavier, slower.
As to whether you notice any of this on the road I would have my doubts - I can't - but you certainly can on the mech stand side by side.
FWIW the only improvement Ultegra brings over 105 that I really miss is the bonus buttons on the shifters.

I have them on two of my bikes. Changed my cranks to 165 last year and these were a cheap way of doing that. They are OK. Not light, not outrageously heavy. Only real problems are that they are very chunky so difficult to get the front mech adjusted properly without the mech hitting the crank. Can fix it with a gxp spacer (couple of pounds).
But I would recommend a Senicx PR3 instead - I also have one of those on a different bike and it is just a little bit better in every way (lighter, miles more front mech clearance).

I have used the Elita One carbon seat clamp and I would not recommend it.

It states on it that you should tighten to a max 6nm which I duly did and it was way too little to stop my seat post from moving. I attempted to slowly increased the torque past that (to a still not particularly high 7nm) and it snapped. So clearly a proper weight weenie item right at the limits, use at your own risk etc.

Which would be fine but for the fact it was only 3grams lighter than the stock alloy clamp it replaced.

IMO the gains are way too small to be worth going weight weenie here.

I have 165s and they definitely do not have a comically large amount of metal sticking out past the hole (which would be the case based on your theory bearing in mind they are also sold as 175s)).

If you mean modern Trek Madone style, I have the Panda Podium x Ahyka set for my Winspace and they are fantastic. Really clean design, integrate well with the frame, well made and the black with gloss logos looks really premium. Because I liked them so much I got a second set for another bike - but this time just the Ahyka bottles from AliExpress. And even though ostensibly they are the same, they just look and feel cheaper. I'd have no hesitation to recommend the Panda Podium versions, and not sure if the AliEx versions were fakes, but the cost difference was small enough if I wanted a second set I would just go back to Panda Podium.

If you want a more traditional Elite Chrono style TT bottle, I've got an original Elite, and an AliEx clone that works using magnets, and I prefer the AliEx version. The cage on the Elite never worked that well - the magnet is a bit bulkier on the AliEx bottle but an easier process to slot back in the cage. They don't hold very much, and really only integrate well with the down tube meaning they look a bit off when using two, so I think they are more suitable for TTs where you are more interested in using the bottle as a fairing rather than drinking very much out of it.

Whilst there is a wealth of information here https://chinertown.com/index.php/topic,3121.0.html I think the base YKYW bibs are hard to beat for the price.

Whilst I know designed for use outside I don't think I would personally because they are quite open mesh, which makes them very light and airy for the turbo. And you can often get for £15 / 20 euros / dollars (though tariffs may have changed that last one). I could only see on 'Bundle for less' ATM so can't paste an easy link.

I now have three bikes with Shimano 12 speed, one each with full 105, Ultegra and Dura Ace, so feel well qualified to anwer this!

Functionally 105 is absolutely fine, there is very little difference between it and the others there are just these things that IMO is well worth going higher up the range:
- The shifters don't have the bonus buttons at the top, which once you have and have set up to control your computer, you will miss massively. You can set up Front Shift Next and get a free button back on 105 to control using tap, double tap, long press - but its nowhere near as good
- The rear mech on 105 has bushings in the jockey wheels. Which feels ludicrously cheap in a groupset that costs about £900 / 1000 dollars / euros. Very quick and cheap fix in paying for Ultegra jockey wheels, but you shouldn't need to do that

Then...
- As mentioned, the front mech for 105 is a big bulbous thing, whereas the Ultegra and Dura Ace look much smaller and neater, if that bothers you. They also work faster, but you won't notice that unless bench testing and specifically looking for it
- 105 cassettes are absolutely fine and work just as well as all the others, as long as you don't want 11-30. And if you wanted 11-36, you need 105. And all work with all, whatever Shimano says

And IMO these things Shimano have fallen behind aftermarket parts here:
- Cranksets. Shimano chainrings are lovely things and the front shifting is super slick, but it should not be possible to save c200g off a top of the range Dura Ace crankset with an aftermarket unit. I changed my Ultegra for a Leese when I moved to 165 cranks, keeping the original rings, which is a good compromise
- Rotors. I know some people swear by Ice Tech but I can't get on with anything above 105 level, the CL800/900s just seem to go out of true far too quickly for my liking

So if it were me... I would buy a 105 groupset, change the shifters for Ultegra, swap the jockey rings out of the 105 mech for Ultegra. And if at some point you want to save weight you can buy a carbon crankset and monoblock Spedao / Goldix cassette, and you'll end up with something lighter than Ultegra anyway.