Those would be considered very marginal. The best plates are titanium, and the second best are made of ceramics, but they can only take a couple of hits before they begin to degrade.
The point is, it's always better just to not get hit. Put on titanium body armor and I'll bet I can find a DU round that will go right through it. Nobody's ever safe just because they wear ballistic vests.
Never said they were. I don't know of an DU small arms ammo, but the SLAP rounds are out there, and .50 cal will punch a hole right through it. But, as you said, the goal is simply to not get hit in the first place!
Fired from a .50 BMG, I heard, but that might be more legend than truth.
The smallest DU rounds listed in the US Army Ordnance data sheets are 25mm. The SLAP rounds are tungsten. Once you get to a certain point tungsten is just as effective as the DU with none of the environmental concerns.
From my research, 20mm is the smallest, but the point is that it has to be a sabot round, the DU being a uranium dart that separates from the rest of the bullet. .50 BMG is capable of firing such a round. This might interest you:
.50 BMG Flechette rifle - The Firearm Blog
It actually fires a DU round, but from the article it seems this was a prototype and not currently in production.