As much as I love Cloyster, I don't beleive he'll end up in OU, even with such fantastic buffs.
With the reduction in the availability of Stealth Rock, (People will be wanting Dream World abilities too much to trade up from D/P), two things happen to Cloyster:
1. More flying types for him to destroy
2. Rapid Spin isn't so vital.
Unfortunately, the boost in the viability of several flying types also influences potentially better Ice types, like Mamoswine and Weavile, both of which don't require a boost to reach good levels of attack and speed, and can run Choice Sets better.
Nonetheless, a +2 Attack Cloyster will easily have a more powerful Ice Shard than either of the above and will also have access to the most powerful Physical Ice attack (I beleive there may be a better attack than Icicle Spear, but is a signiture move of a legendary).
Cloyster's downfall also comes in the form of poor type coverage.
His strongest attacks are Rock and Ice, which together don't offer much, bar completely breaking flying types, and he must also sacrifice a move slot for his new boost, meaning he cant carry all of his utility such as Rapid Spin and Explosion in addition to his offensive abilities.
Cloyster also gets completely broken by Priority.
Lucario, Infernape and Breloom, 3 extremely dangerous offensive threats, all have super effective Priority moves which are far more danagerous after cloyster lowers his own defense, not to mention that two of those resist Cloyster's own Priority attack, should he run it.
Although I see cloyster being used occaisionally for a "Surprise! Your annoying Zapdos is about to die!" or maybe to destroy an Ononokusu with Ice Shard while rapid spinning (Even thogh Mamoswine and Weavile do it better, they just don't have Rapid Spin) I don't think he'll enter mainstream OU.
Worst case scenario is that this guy gets trapped in BL for being able to get off a boost abusing his monstrous defense too easily in UU and then sweeping with Icicle Spear.