If special Victini were to lose power on its main stab move, lose an insane amount of its movepool, lose its versatility, lose its bulk, and only gain enough speed to outspeed lando and Thundurus-T, would it be OU in a more competitive gen? That's Delphox. It's preposterous not to think it wouldn't be OU and even if it did make the cut by some miracle, it would get outclassed by the lower tier Victini
To begin with: your premise is flawed based on the fact that there is no OU tier yet.
And outside of that, Delphox still gets calm mind and magician, and it's speed matters beyond just 'oh it outruns the things in-between this speed stat and this speed stat'. And it does have superior special attack which matters beyond just blue flare vs. fire blast. There are reasons to use it in place of Victini.