351 is the magic number for offensive pokes in ou, it puts you above the base 110s and everything that isnt scarfed, talonflame, greninja, or deoxys-s. this is why thundurus-i at base 111 has such an annoying speed tier, because max plus it hits 353.
when it comes to speed tiering scarfers, with rotom-w ill always run modest every time because the gap between max modest and max timid (406-447) has literally nothing useful in it except adamant scarf lando-t, while scarfchomp is a toss-up between adamant and jolly for whether you need to revenge kill zard x at +1. for the same reasons its usually optimal to run adamant natures on speed boosting sweepers like DD mega gyara and dragonite. mega gyara running adamant at +1 speed will be outsped by mega manectric if it already mevod earlier in the match though, so something to look out for.
personally i would almost always run max speed on everything between 110 and 100, the list of big ou threats in here is pretty significant.
Technically 105 and up need max speed always. This includes Mega Pinsir, Galvantula, Infernape and the musketeers. Although like it was said, if you're insanely faster than Greninja, like the few Megas and Deoxys-S, then you obviously have some liberty in regards to what you choose to outspeed. Deoxys for example
could run max speed on the off chance of only speed tying with one pokemon, itself. This is only really worth considering though if with different nature or EVs you can definitively net new one OHKO/2HKO or survive something after hazards or something. It a matter of probability. Are you more likely to need to speed tie other things, or are you more likely to need that particular extra damage/defense?
As I've said, there's this nebulous gray area for everything between Dragonite (base 80) and Garchomp (base 102). Everything outside of this range
tends to run min or max speed, and is easier to account for. While base 100 used to be the old cutoff where you HAD to run Jolly/Timid from the sheer amount of pokemon in that tier, actual sweepers in that range are becoming far less common. It's because the bulk and offensive stats for pokemon in this range vary wildly. Pokemon like Landorus-T, Rotom-W, Kyurem-B, Mew and Celebi have great bulk and defensive abilities where more bulk benefits more than speed. And ironically, the more people acknowledge and understand this trend, the more likely it is to perpetuate. CharX has bulky sets with WoW and Roost or simply Adamant with Dragon Dance. Staraptor likes Adamant with Choice Band for wall breaking. Mew can often go bulky for unique Defog sets. Dragonite with Multiscale and Extremespeed can often afford not to go Jolly with the option of Dragon Dance. This area, this "speed pocket", is where I see speed fluctuate the most and is still the hardest to account for. I STILL don't know how to EV my KyuB I'm considering for these reasons.
Below this is the "speed creep" zone, where most things either run no speed, or only enough to bypass a significant obstacle, usually within 10 base speed points. Heatran for example sits at an unusual base 77. With just a little investment it can outspeed Mega Venusaur at base 80, which is not a bad idea if you want to burn it or get up Substitute before a possible Leech Seed/Sleep Powder. Mega Heracross is below him, either 70 or 75 offhand can't quite remember. But the point is whoever goes first in this matchup will easily win, so Mega Heracross often can not afford to be slower than him.
And than there is the anomaly Aegislash. This creature has fucked up how we think about speed entirely. Because would be checks for him
entirely depend on if you attack after he does because of his ability, which is really awkward. It's why checks in Aegislash' range often consider running negative speed to go after him! That's relatively unprecedented, barring Trick Room teams.