Flygon is still weak by the current metagame standards. It will have a hard time fitting in regardless.
People said the same thing back when Garchomp was banned in gen IV. Gliscor and Mamoswine were considered to be superior replacements for an offensive ground type back then, yet we all know where he (Flygon) eventually ended.
That being said I don't think Flygon will rise too much, if at all, in the current gen. At very best he will end up in the lower parts of OU with Tornadus and Venusaur... that is, unless some popular user starts making dozens of threads titled "Flygon is awesome" which will push him up in the usage ranks like in good ol' gen 4 OU, lol.
80 base special attack is pathetic.
Garchomp could run that exact set (with Surf replacing U-Turn) and it was still considered a gimmick set because of the pathetic damage it dealt.
I doubt it can even 2HKO Ferrothorn in the Rain with Fire Blast.
If you want to use Flygon, go physical or don't even bother.
Landorus can't beat Ferro, at all. Scizor sets up on it, and Rotom beats choice sets?
Assuming you have +2, assuming Sand, assuming LO, Landorus can beat Ferro. Most Landorus sets are choice, and Flygon doesn't need to be on a sand team. If Flygon has a niche this gen, it's that set.
If Excadrill goes to UBERS and Garchomp doesn't go to OU then Flygon will definitely go to OU just like in Gen 4 but Sczior is OHKO by Fire punch 100%
-_______- My point is that Landorus is usually better, but if Flygon has anything this gen to differentiate itself, it's his movepool
Mixmence can run a better set with Draco meteor/Fire blast/outrage/earthquake though.