Surely the "does weight matter" thing is one of the easiest things you can test at home? Get a non transparent water bottle, that you can't see through, and get your partner to either fill it, or leave it empty, but not tell you. Stick that on the bike, do a 10k circuit. Recover, do the same circuit with the contents of the bottle inverted (fill if empty, emptied if filled). See if you notice the 500g difference...
There is a GCN video where they add 2kg to a bike and do controlled runs up a climb, to see what the difference is. Can't remember what the result was off the top of my head. But worth a watch.
There's comments about amateurs worrying about 50g on their cassette, or saddle etc... If you have a bike with 20 components, and you can find 50g saving on each one, you have saved 1kg. I find this with Ultralight Hiking. Sure I'm only saving 50g here, or 100g there, but over all I've gone from 16kg to 5kg, so it's been worth it.
On the disk brake thing, I'm a complete convert to them. Allows me to change wheels/tyres around a lot easier, The flexibility is worth any weight penalty as far as I'm concerned.
Dunno if it applies to high level pro bikes, but in things like Formula 1, they cars actually come in under the weight limit as standard, and so the teams add weight in certain places to move the balance around and improve handling. Can the pro's do this?
J