I agree with alexwolf that a speed-boosting ability or a natural means to outspeed common Choice Scarf users is more important than priority. CAP6 would prefer not to have to use moves with poor coverage and 40-80 BP. We all know there are ways to make Choice Scarf opponents much less threatening, but since I don't want to polljump I'll leave it at that.
Now, there are two schools of thought when designing a Belly Drum Pokemon. We can either build a Belly Drummer whose goal is to defeat offensive teams while being more vulnerable to defensive teams, or we can make a Belly Drummer that greatly threatens defensive teams while struggling against offensive teams. Defensive teams will try to defeat CAP6 by walling it, phazing it, or wearing it down with residual damage. Offensive teams will try to defeat CAP6 by attacking it while it sets up or revenge killing it. It makes sense, therefore, to focus on choosing either to be walled by common walls OR to be revenged by common revenge killers, but not both.
The question is: which one do we want to defeat? Defensive threats such as Skarmory, Hippowdon, and even Ferrothorn seem designed to take this Pokemon's hits and counter with a Roar, Whirlwind, or even Leech Seed and Protect. On the other hand, speedsters like Scarf Terrakion, Keldeo, and non-Scarf Jolteon exist in this metagame for the purpose of shutting down Physical sweepers and limiting the number of Pokemon they can take out.
Now, it is definitely possible to take down either or both of those groups if we wanted to. However, making a Belly Drummer that can easily take down offense and defense will be broken without question. The question is: do we target offensive teams or defensive teams? I believe the typing we chose, Flying/Steel, is conducive to defeating defensive teams while being more susceptible to revenge killing. When we debated Steel/Ghost versus Steel/Flying, we chose the offensive prowess of the Flying type and the Earthquake immunity over the Normal immunity, Fighting immunity, STAB Shadow Sneak, and Ice resistance. This makes CAP6 more vulnerable to incoming priority while giving it neutral STAB against the likes of Forretress, Ferrothorn, and overall more powerful attacks. We wanted more powerful STAB and better coverage rather than immunity to Mach Punch and ExtremeSpeed and a resistance to Ice Shard. Instead of countering offensive teams by trying to outspeed priority users, we should play to our strengths, namely, the potential to have powerful STABs and the great neutral coverage Flying has.
If we target defensive teams, which inherently are easier to set up against than offensive teams, then we should counterbalance the advantage we have against defensive teams with a susceptibility to common counters to set-up sweepers on offensive teams. Specifically, I'm taking about priority attacks and Choice Scarf users. I believe that few, if any, common walls should threaten CAP6. CAP should threaten few, if any, revenge killers.
Specifically:
Threaten
-Steel-types. CAP6 should threaten Steel-types because most Steel types are used for defensive purposes. Although Steel resists both Flying and Steel, the most popular Steel-types in the tier are Ferrothorn and Scizor. Neither of those Pokemon resist Flying. We don't want to be walled by Skarmory when we naturally do well against so many defensive Pokemon because it makes us much more susceptible to stall.
-Ground-types. We're naturally immune to their STAB, most Ground-types cannot hit us super-effectively, and only Hippowdon and (to an extent) Garchomp even have recourse against us. If we can find a way around being phazed then Ground types become the best Pokemon to set up against.
-Mono-attacking Water-types. No, these Pokemon are not common. Yes, they pose a great setup opportunity. Tentacruel, defensive Politoed, and Vaporeon should not threaten us severely and serve as great fodder to set up against.
Be (Specifically) Threatened By
-Choice Scarf Latios, Latias, Jolteon, and Starmie. The best of the best revenge killers should be able to take us down.
-Mamoswine, Breloom, and Choice Band Dragonite- These powerful priority Pokemon are offense's best answer to fast Physical sweepers. We should not be going out of our way to defeat. I don't believe these three Pokemon should come in and clean us out, but each should be able to revenge kill us well enough.
This isn't an exhaustive list, but it's not the most important part of this post. The best way to take down CAP6 should be to attack it as it sets up, then finish it off with some priority if need be. If CAP6 can set up against a team that has its priority attackers taken out, then honestly it deserves to win the game right there. This isn't a simple case of "remove counters, set up" like most setup sweepers are in OU. This is "remove revenge killers, sacrifice 50% of your HP, take a hit while doing so, sweep", and it's fair to expect victory once those criteria have been met. Furthermore, two Pokemon in particular, Ferrothorn and Garchomp, have abilities that can shave off the last of CAP6's HP. If the opponent has no fast Scarfers or priority users left (or at all) and allows a Belly Drummer to set up against them, they do not deserve to win the game. Specifically...
Any Pokemon that can do 50% to CAP6 with any move immediately threatens CAP6 and prevents it from setting up. We are literally "checked" or threatened by about 95% of the OU tier.