Rock Polish T-Tar is a pretty good late game sweeper in Ubers. His main issue is with the slower, bulkier Ubers, Kyogre, Groudon, and Dialga.
A set I've been using:
Tyranitar @ Expert Belt/Life Orb/Leftovers
Sand Stream
4 hp/252 Atk/252 Spd
Adamant
-Rock Polish
-Crunch
-Stone Edge
-Dragon Claw
With Max Speed and an Adamant nature, Tyranitar can even uutspeed Deoxys-A with 442 speed. The combination of Crunch, Stone Edge, and Dragon hits most ubers super effective and all of them at least Neutral. What makes this set really appealing is STAB Crunch for the huge number of psychic pokemon out there, and the fact that Tyranitar can outspeed and deliver a STAB super effective Stone Edge against Lugia. Most of the Dragon Types are relatively frail, and Dragon Claw delivers a punishing blow to most of them.
Why not max speed with a neutral nature? That would let T-Tar hit 484 Speed, outspeeding +speed natured, scarfed based 90s, Dialga, Groudon, and Kyogre. Of the three, Tyranitar can only run Earthquake to hit Dialga super effective. The other two are quite bulky, and can seriously hurt Tyranitar with super effective STAB surfs and earthquakes. Scizor is also a severe problem, with his powerful STAB bullet punch. And unfortunately, you can't outspeed ScarfChomp and Scarf Palkia, both of whom will be serious problems with their vicious STAB super effective attacks.
Despite the fact that he can't seriously hurt these slower pokemon, he is definitely a huge threat later in the game when he can outspeed most ubers and deliver enormously power super effective hits.
Item choice isn't too tough. I'd say expert belt is best since you do hit so many ubers super effective with this set. LO is useful against those pokemon you don't hit super effective against, though the recoil damage is not too bad since you do outspeed so many pokemon. Leftovers is probably unnecessary since you should be able to sweep very effectively late game and avoid most damage.
I think the problem with Rock Polish Tyranitar in OU is that he lacks the punch for walls like Swampert and Hippowdon that aren't found in Ubers. Swampert especially is the main problem that a Rock Polish Tyranitar has, and HP Grass would be a pretty dumb idea. Meanwhile, most uber pokemon are weak to Tyranitars attacks and have a weaker defense. Oh and I forgot Scizor is so prevalent in OU, the whole outspeed thing is not all that useful.