Hi. You could apply some kind of half-half-half-half-.... rule.
Reed: 50%
Mouthpiece: 25%
Neck: 12.5%
Horn: 6.75%
I keep this in mind just to stay realistic.
As a longtime owner of a generally overlooked horn, the MkVII tenor, I could very clearly feel the improvement by changing the neck. 2 times.
2003: Ref54 neck, after testing Ref36 & Ref54, S-III and Ponzol, 2 of each. Easier response all over the range, and a livelier horn. I could feel the difference in the fingers. I've played it like this until about a year ago.
2022: S-III neck, another step easier response in the low end at low volume, one of the key challenges of the MkVII, a horn designed to be played loud.
Speaking of bank account, both are standard Selmer necks, so nothing exotic. The latest S-III comes from the used market, I could sell it break-even.
The MkVII is a great saver: you get the roar of a King S20, the Selmer sparkle and an almost Yamaha intonation and build quality for the price of a used Mauriat.
I did some "neck swapping" for fun on alto and soprano, it had less effect, probably due to the relative shortness, and maybe the simpler geometry. I keep playing those horns with the stock neck. I could also be less demanding on the small horns.
On baritone, I bought a "pink brass" original Selmer neck, and keep playing it. I'm not sure if it really changes anything, I assume it is pure coquetry.
Voilà.
I recently started I thread regarding Yamaha tenor necks, as there is quite some fuss around the V1, the C1, the M1 and maybe the ZZ or XXL, some brass, others solid silver. The buzz is fueled by Yamaha themselves, having all of them in their product list.
I know it can make a difference, but keep in mind it is somewhere around 12.5%...
12.5% of what ? Oh man, don't ask me.... Certainly more about response and intonation than sound.
Edit: 2003, I was 40++. Highschool ? Do they really know what they are doing ?