Darkblader You're stretching yourself
really thin with that set. Not only are you unable to fully capitalize on the bulk AV affords with the -SpD nature, but you can't even invest in both attacking stats and still have enough EVs left over for good bulk and/or speed. Patching weaknesses is nice in theory, but doesn't matter in the end if your mon isn't actually
good at anything. The set you posted doesn't excel as a mixed attacker (no power-boosting item like LO/Dark Plate/Sky Plate). It also doesn't excel defensively (each point of SpD lost throuh Rash/poor IVs is actually 1.5 points lost with AV, plus it can't even Roost like other defensive sets). And finally, despite possessing (inconsistent) priority, it's probably slow as balls otherwise (or you skimped out on defensive investment).
Maybe Rash could see some use on a mixed attacker over something like Hasty. It's not a bad idea. Please, just...don't use AV with Rash. If you really want to use AV, go Calm (more SpD) or Modest (more damage output while retaining your defenses). Like you've said, you've still got Foul Play if you absolutely *need* a physical attack. The difference is that Foul Play doesn't force you to compromise your other strengths (Def/SpD on a set with a defensive item). I
do think AV Yveltal is kind of neat conceptually because of Oblivion Wing (no other AV user possesses such potent self-healing). Just don't get suckered into not specializing just because you've got a versatile Pokemon.
As far as that catch, the SpD is still kind of mediocre (although everything else is awesome). For Yveltal, HP is probably the least important stat to have good IVs in because it's so high (each IV = less % overall). If you're patient, or if you're more set on AV than a Rash nature, keep resetting. If not, I suppose it'd still be decent as an offensive mixed attacker.