Thats what I looked at from a single target BW perspective. As a rotation I chose Ignite -> Slow Boil -> Boiling Blood -> Pyroclastic Surge -> Sear -> FBB (2 ticks) -> Nova. It's probably the hardest hitting and also longest rotation a single target BW can do. As tactics I chose Endless Knowledge, Burn Through, Fuel to the Fire. That would leave space for Flashfire which I currenlt cant simulate.
Target was a healer with max deft defender speced.
The Figure below shows average damage each ability did with 1 million calculations.

As can be seen the conquerer set results in slightly higher damage for all abilities.
Thoughness debuffs are fairly frequent therefore making the thoughness proc from the conquerer set relatively useless.

Comparing the two sets under the same 100 thoughness debuff results in no change of the previous observation. The conquerer set is still better for damage dealing.
The reasons are also fairly simple: The Onslaught set has less critical hit rate and less disrupt strike, the proc just cant make up for it in a single target regime.
Or you could of course see it positive. The Onslaught set is almost on the level of conquerer but (probably) can be aquired through pve. From this point of few it appears to be well balanced.