We can discount anything that took place while the Army of the P was on the Peninsula, for starters.
I'm not so sure we can. Wittenberg said:
"On the Peninsula McClellan parceled out his volunteer cavalry regiments to specific infantry brigades, primarily using the horsemen as messengers and orderlies. This was a poor use for an expensive arm of the service like cavalry ... McClellan frittered them away."
But when I asked him for data about this he refused to provide any.
Obviously, if this is a statement which is not well supported by evidence (and if in fact the Army of the Potomac's cavalry was largely concentrated and used appropriately to screen the flanks, provide a reserve striking force and conduct recces) then the idea of the Army of the Potomac's cavalry having a low standard of performance is not well established.
So are we proposing a laudable performance in Maryland or Loudoun?
Well, the Union cavalry did well in both campaigns at the scouting and screening role, driving Stuart's cavalry back through the Catoctins in Maryland and fighting multiple (successful) cavalry actions during the march south in the Loudoun Valley campaign. In particular the Union cavalry takes three guns from Stuart's horse artillery, which isn't something to sneeze at.
The scouting and screening in Loudoun is particularly worth noting, because there is a clear information disparity between the two sides.
On or about the 9th of November Lee's sense of where the Union army is is wrong by about fifteen to twenty miles (that is, he tells Jackson that the Union army is around Piedmont Depot and Rectortown, 35 miles from Culpeper; the Union army is actually mostly at Warrenton, 20 miles from Rectortown and 22 miles from Culpeper, and the 9th Corps is at Waterloo only sixteen miles from Culpeper). Conversely, the Union cavalry has a pretty good idea where most of Lee's army is (the details shift around slightly but for example they confirm on the 8th that Jackson and both Hills are still in the Shenandoah and are clearly aware that Longstreet is at Culpeper).