My co worker was struck by 3 rounds of AP Russian steel core from a PK at close range in his paracleet armor, and it stopped all of them on the same plate insert even though they were in very, very close proximity. There are many, many cases out there in google land of the plates absorbing multiple hits. They don't shatter into pieces, not at all.
I've got some vids I'll put up from BW's armor vs ammo shoot in 2006 or so, you can see various AP rounds being defeated by the plates, and many were tested with multiple hits.
Like I said, 5.7 or any pistol round for that matter has no chance of defeating most modern armor, and even older 3a level stuff it usually won't defeat with any regularity. Soft armor of the 2 to 3 class, yes, it'll defeat it, but so will 9mm 75 gr RUAG frangible going 1600 fps, both front and back panels.
Making a decision to buy 5.7 if the expectation or reasoning like you first were saying is to defeat the modern hard armor the Chinese and virtually every other army is trying to equip its troops with is a fail, as it can't, no pistol round can. Like I said, it will breech other barriers that many common pistol rounds can't, but vs lvl 4 armor - not a chance.
There are piles of vids of cheap 200$ or less plates taking multiple 30 cal rifle hits at point blank range and still stopping them all. https://www.youtube.com/watch?v=1HRGpQ6-rz8 is just a single example, there are dozens out there. In this vid 2 308 rounds are fired into the exact same hole - the plate still stops the 2nd round. And this is cheap stuff compared to Dragon scale or Paracleet systems. When you say I underestimate 5.7, I think you're way overestimating it. 762 Nato will vastly outperform 5.7 in every measurable way in terms of penetration, yet even cheap plates will stop it, so how does 5.7 have a prayer out of a pistol barrel at that limited velocity? Even from a P90 or rifle, 57 is no better than an SS109 556 Nato round in terms of penetrating power. All 5.7 was designed to do was defeat light soft body armor that was designed to stop 9,40,and 45 from a pistol or short barrel shoulder supported weapon. And that's it, it is no wonder round with some special AP component that will blast through lvl 4 armor. Again, it can't even defeat 3a soft armor with the AP round, and has no chance with the standard FMJ projectile.