Cleric Sylveon would be a superior Cleric Florges in UU in practically every way, outright to the point where Sylveon's cleric set will almost completely invalidate Florges in UU.
How is Sylveon an upgrade over Florges? It passes bigger wishes, but if that's all I wanted why wouldn't I just use Vaporeon? It hits a bit harder with Hyper Voice, but it's also a lot slower. It would be stuck between a rock and hard place.
This was a hotly debated topic back in OU two years ago as to my knowledge, but basically, Sylveon's cleric set is a completely superior version of cleric Florges. In fact, Florges has nothing over Sylveon in terms of being a cleric, and Sylveon is superior in every aspect.
Sylveon passes bigger wishes than Florges, and that is a huge deal. You can't say "if I want bigger Wishes then I'd use Alomomola" or something, since Sylveon replaces Florges as a Fairy-type cleric, not just as a general cleric. Sylveon has the Fairy-typing, which gives it the advantage over popular threats such as Salamence and Hydreigon, just like Florges does now. Sylveon passing bigger Wishes makes it a vastly superior cleric to Florges, meaning it takes over as a Fairy-type on most teams as it can heal more HP to teammates than Florges can, which can make a big difference in some matches. By using Florges over Sylveon, you are putting yourself at a disadvantage since you pass smaller wishes and it's overall worse as the Fairy-type on a team.
Sylveon's bigger wishes are what make it much better as a cleric than Florges, but it also has similar bulk to Florges, which further invalidates Florges's use as a cleric due to Sylveon's potential existence. Let's look at the calcs, first in physical bulk:
252 Atk Earth Plate Krookodile Earthquake vs. 252 HP / 252+ Def Sylveon: 171-202 (43.4 - 51.2%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Earth Plate Krookodile Earthquake vs. 252 HP / 252+ Def Florges: 168-198 (46.6 - 55%) -- 10.9% chance to 2HKO after Leftovers recovery
As you can see, Sylveon is slightly bulkier than Florges and since it passes bigger wishes, Sylveon has a complete advantage. It takes hits just as well as Florges does and passes bigger Wishes, all the while performing Florges's role as a defensive Fairy-type on teams. The same applies to the special side:
252 SpA Life Orb Heliolisk Thunderbolt vs. 252 HP / 4 SpD Sylveon: 134-160 (34 - 40.6%) -- 44.1% chance to 3HKO after Leftovers recovery
252 SpA Life Orb Heliolisk Thunderbolt vs. 252 HP / 4 SpD Florges: 117-138 (32.5 - 38.3%) -- 2.2% chance to 3HKO after Leftovers recovery
Okay, Sylveon takes the hits
slightly worse than Florges does on the special side, but since it's only a 2% difference, the difference in damage on the special side is frankly negligible. They get 3HKOed and 2HKOed by the same things, and they take similar damage on both the physical and special ends of attacks, and Sylveon passes larger Wishes, which thereby invalidates Florges as a Fairy-type defensive support Pokemon. There's also the fact that Sylveon hits harder than Florges, which can make a difference in taking down Pokemon.
0 SpA Florges Moonblast vs. 76 HP / 0 SpD Mega Abomasnow: 109-129 (32 - 37.9%) -- 92.1% chance to 3HKO
0 SpA Pixilate Sylveon Hyper Voice vs. 76 HP / 0 SpD Mega Abomasnow: 132-156 (38.8 - 45.8%) -- guaranteed 3HKO
Here, Sylveon's Hyper Voice
clearly outdamages Florges's Moonblast. Even the
minimum damage output of Sylveon's Hyper Voice is stronger than the
maximum damage output that Florges's Moonblast can muster, which makes it more capable of dealing damage. This calc here might not seem like much, but it matters when you count in Stealth Rock, which is usually (and should be) on the field in most matches. Sylveon's Hyper Voice also has the luxury of circumventing Substitute, which is extremely useful to ensure a defensive Pokemon like Sylveon isn't as easy of setup fodder as other defensive mons.
You cannot use Speed as much of an argument to Sylveon vs. Florges since Speed is extremely irrelevant to both Pokemon due to the specific way in which they play. They are both defensive Pokemon and won't invest into speed very much anyways, and focus on HP. Florges doesn't outspeed anything that Sylveon doesn't in the UU metagame except for like Umbreon and Chesnaught, both of which pose zero threat to Sylveon in the first place.
So basically, Sylveon is a complete upgrade over Florges since it does everything Florges does, but better. It passes bigger Wishes and hits harder: those two things are enough to make Sylveon flat out superior to Florges. Compared to Sylveon, Florges is weaker, is not quite as bulky, is a worse cleric than Sylveon since it passes smaller Wishes, and though it is faster, that is irrelevant and will never matter due to the specific way in which they play.
To sum it all up, Sylveon is not only an upgrade over Florges, but it outright invalidates Florges as a cleric. Due to the reasons I stated above, Cleric Florges, assuming Sylveon ends up in UU, will become
completely outclassed, and there will be absolutely zero reason to use Florges as a cleric if Sylveon falls to UU. Considering Florges is very popular as a cleric right now, Sylveon would be a
very popular option on the kinds of teams that use Florges (which is frankly a lot of them, meaning Sylveon will be both common and good).
Specs Sylveon would be decent as well, since Specs Hyper Voice hits crazy hard, and Sylveon can afford to invest in bulk and dry Baton Pass. Gardevoir would be competition for it, but Gardevoir is insanely frail and is kind of easy to revenge kill. Sylveon would have the advantage by being a bulkier alternative that also hits hard but also has dry Baton Pass, and checks Salamence and Hydreigon slightly more effectively (Hydreigon especially since Sylv actually resists Dark Pulse).
I won't say anything about CM Florges though since that might keep it relevant to a degree, but likely not enough to keep Florges in UU (it'll probably fall to RU if Sylv drops, though it would be good there since offensive Florges would actually be plausible there).