This might not be the kind of answer you're looking for, but most IDPA/IPSC competitors who still run factory ammo shoot whatever's cheapest!
IDPA certainly, and USPSA 99% of the time does not require -- and indeed, doesn't give you the time to capitalize on -- the slight differences in accuracy potential between one brand over another. As long as they are reliable in your gun and both hit pretty close to your point of aim, there isn't a compelling reason to buy Federal Gold Medal Pistol Match over Win White Box, accuracy wise. Most production handguns will shoot at least most types of factory ammo into a 4" group at 25 yds. (If the shooter is up to it.) That's plenty good enough to hit 6" A (or "Down 0") zones at IDPA/IPSC distances.
Anyone who blames their misses on poor ammo accuracy will get laughed off the range!
Most shooters still buying the factory stuff will have run some Rem, some Win White Box, some Federal, some PMC, and probably Blazer and Wolf, too.
As long as they function reliably in your gun (which is far more important than the nth degree of accuracy) run whatever you can get your hands on for the least money.
Now, a smart competitor will have tested each load to make sure that it impacts pretty close to his point of aim with the gun he/she'll use. And each one may have picked a personal favorite which they'll usually buy if they can find it reasonably. But I don't think anyone can make a blanket statement that (Insert Commercial Ammo Brand Name Here) is not accurate enough for IDPA/IPSC, or that (Insert Other Brand Name Here) is
the best.
But what do I know? The only factory ammo I've shot in competition this year were 72 rounds of mixed Blazer and Silver Bear 9mm that someone gave me. It worked just fine, too!
-Sam