Unless there's a very specific and notable OHKO, I don't see why you would ever use Meteor Assault over Close Combat, which isn't all that much weaker.
Yeah, like I was sayin' I know it typically sucks, but there's a big difference between theorizing on paper and things actually in motion. For one example, last gen on my charizard-y I ran air slash, solar beam, fire blast, and blast burn. In that case it was more of not really needing anything else for that last slot that my team couldn't already cover/handle. And I had many moments where I'd hit with fire blast as my best option, but deal only 40-45% against a boosted foe, then take a hit and nearly die myself. Now, typically speaking that would be game over for me, but with blast burn I was able to win. (In fact, in Ultra Sun I never lost a single match, granted my team was built and tested prior in Moon, and I didn't play Ultra Sun for long, but it was still a strong record) Is Metero Assault Gimmicky? Yeah, totally, :) but he's not going to be top tier anyway, so I built him the way I described and have been happy with the builds success.
The only other time Focus Energy + Leek could be better is against defense boosters, which might see more use this gen because of Body Press, but besides Cotton Guard Dubwool, most of the defense boosters I can think of destroy Sirfetch'd either way (like Bulk Up Corviknight) or get destroyed by Sirfetch'd either way (like Bulk Up Obstagoon).
I wouldn't use this idea over my first one either, but yeah defense boosters was the niche I had in mind as well. As far as something like the standard bulk up corviknight from the strategy dex though, while close combat is guarranteed 2 hit KO, but meteor assault can OHKO, and sirfetch'd is faster, and brave bird might not even kill, so you could try to sneak in a night slash for ~30% before finishing it off with meteor assault, a feat which close combat could not.
252+ Atk Sirfetch'd Meteor Assault vs. 252 HP / 32 Def Corviknight on a critical hit: 385-454 (96.2 - 113.5%) -- 75% chance to OHKO
Any hoo, just some thoughts.