I don't agree with Sylveon going into A+, let alone A+. A is fine where it is.
Sylveon is (still) quite the one-and-a-half trick pony. Any team capable of prepping for the specs set is fully capable of handling the cleric set; the difference in power and lack of coverage is staggering. As a cleric actually, having used it and played against it, I've found Sylveon exceptionally underwhelming. It does the same thing Umbreon does (which thanks to much better bulk and slightly higher Speed, does the cleric thing better imo). The only trade-off is Hyper Voice and Fairy STAB on it's own isn't a lot to write home about. Without Choice Specs, it's nowhere near as terrifying. Things like Nidoqueen or Tentacruel are now conditionally viable switch-ins, since you're either unable to 1v1 them or you drop Protect and lose thanks to Wish's one turn delay. Baton Pass has a similar effect here, and unless you BP without passing a Wish (ala pivot), then the same thing happens.
On a side note, running BP keeps Sylveon from fitting on WP Celebi teams (which has to go S-rank if Salamence gets nerfed imho, just for the influence these teams will have). Not an issue at all, but something I found curious. Baton Pass clause is silly.
So if the cleric set is okay, but still underwhelming, then your nomination rides on the Choice Specs set. The boosted Hyper Voice is a monster sure, but now you have no defensive utility outside of keeping Hydreigon from spamming. And many of the things that you 2HKO can still switch-in and threaten Sylveon. You need opposing Entei, Steel-types, and/or Poison-types removed before Sylveon reaches its potential. If you nab a Poison-type with Psyshock, you're now a free turn for a ton of threats in the metagame. The same goes for Shadow Ball and HP Fire. Without the max bulk and any recovery, chip damage becomes enemy number one for Sylveon, since threats you'd normally be fine against, namely Sharpedo (as it mevo's) can pick you off once you're at ~70%.
I see a lot of "Crawdaunt syndrome" in regards to Sylveon - it's situationally hard to switch in on and it's got a hell of a nuke at its disposal, but it's still fairly easy to play around if you're semi-competent. Sylveon is dirt slow, and unless it's riding Specs, has a limited offensive presence. And if you do run Choice Specs, its defensive presence withers. In a vacuum Sylveon is really good, but in practice it's simply good. A is the perfect rank for it, and as the hype train (slowly) dies down, that should become more apparent.