Considering you can't even play 6v6 on Champions, and we are modify the tier to allow it; I see no reason to keep item clause. At the end of the day this is essentially a pet mod that allows us to play with Megas without the bloat/nonsense of NatDex. So why should we be super strict about keeping it 1 to 1?
By virtue of being an official Pokemon product I don't think the 'Pet Mod' description really makes sense.
The reason why I'd want it kept as 1 to 1 with the official thing as possible (barring Pokemon bans is because there'd be no other point otherwise. The point of a Champions OU (if it comes to pass) would be to
play Pokemon Champions, not a superficially similar format. Showdown tries hard to match catridge accuracy as much as possible so matching Champion's seems like a good priority.
Anyways, I want to talk about some Pokemon that will either be Extremely Annoying or just plain ol' Very Good.

As
G-Luke already said, this mon will be strong as hell. With decent team support it'll probably be one of the strongest megas one could run into and its bulky as hell to back things up. At a glance, it does have 3-4 Pokemon that can slow it down, but two of them are Megas (Chimecho-Mega, Venusaur-Mega
though a +1 Psychic does a ton of damage, Tinkaton, Slowking-galar) and Tinkaton can get worn down overtime. Save me, Slowking-Galar...

No boots means you'll see the two magic bouncers pop up a bunch and Excadrill embarrasses both of them. Can beat Chimecho's ass and revenge-kills Floette-Mega. Plus Hippo's here if you're into that. I expect Excadrill to be
very common.

What if Mega Medicham had 20 extra speed and could use special attacking moves to beat Slowbro

You can laugh but you'll remember this post when your glowking gets one-shotted by a polteageist behind aurora veil. Speaking seriously, though, I really do think that it could be a gigantic threat. The only real defensive counterplay is Umbreon and that's not getting played outside of stall. For offensive counterplay there's Aegislash/Mimikyu and not much else! There's a handful of offensive dark types but they all (a) take a lot of damage from Tera Blast and (b) are weak to rocks! Polteageist will muscle past them without issue imo.

Rocks weakness sucks but its one of the fastest Pokemon in the format and can easily beat Slowking-Galar in a 1v1 with Fire Blast + Psyshock. Really any "fast strong special attacker" is going to go crazy due to the removal of Assault Vest.
From the looks of things the metagame will probably be
extremely offense-oriented even if a lot of Pokemon get banned. Offense just eclipses defense by a country mile imo