So I'm trying to get information and I've noticed that it's close to impossible to find any info on it. Is it a forbidden topic or is it just not a popular plate system?

If you're thinking about buying some I'm afraid thats not going to happen unless you know something/someone I don't. Best I could dig up was an expired ArmsList listing from 2013 selling a supposed XSAPI pair for $1500. Most real, "brand name" SAPIs are, as far as I know, Gov. property. Lots of imitators and plates that are very similar, but getting your hands on actual Mil. issue SAPIs of any kind seems to be difficult. So its not that its a "forbidden topic", its just that it's practically impossible to get your hands on real-deal SAPIs, XSAPI, ESAPI, or otherwise. 

If I were you, I would just look at Lvl. 4 offerings from other manufacturers such as Highcom, Hesco, Paraclete, etc. if Lvl. 4 ceramics are what you're after and forget about chasing after SAPI plates. 

I remember seeing an ebay listing claiming to be an XSAPI before but nothing recently.

Oddly enough there seems to have been a big reduction in the number of Ebay listings for military armor. Not seeing as many for IBAs or black SAPI plates anymore. Wonder if there was a crackdown.

What do you want to know about them? They are heavier, supposed to offer more protection against AP rounds, and there aren't any for sale they were issued in limited quantities and have tan color to them.

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SAPI and ESAPIs can be found with a little digging online, they used to be for sale on eBay in large numbers but they cracked down on it. Both plates are ICW (in conjunction with) plates meaning they require a soft armor backer to provide the full level of protection.  SAPI will provide roughly a NIJ lvl 3 and ESAPI will provide approximately lvl 4. I say approximately as the mil spec plates are not designed or tested to the exact same standards.

Regarding the XSAPI it was developed to address a specific ballistic threat the ESAPI could not stop. For obvious reasons the military did not releas what that threat was or our enemies would use that info against our soldiers. The XSAPIs are built identical to the ESAPI just thinker and heavier. As far as I know the XSAPI was never issued. I'm to lazy to look it up but I remember a number like 40,000 being produced.

I didn't know that the XSAPI ever went into real production. I thought it was just a research project. I could be wrong, because I haven't kept track of it.

It came out and apparently had multiple revisions as well.

A buddy who deployed in 2011 got XSAPIs. When I deployed in 2012 I got ESAPIs. Different locations, different threats, different PPE.

I think the SAPI's I've seen from B.P.M. are contract plates.

Fwiw, current issue is the ESAPI for generic units. I'm sure the operators have cool stuff, but even if I knew, I probably couldn't tell you.

Also, FWIW, even being active duty, currently, it is VERY difficult to acquire, by any means, these plates. They are serialized, thusly the individual must turn in THEIR specific plates, or pepper thine angus.

Afaik, and is printed on the plate, it is good for M2-AP protection. As discovered the hard way by some people I know, they WILL stop lesser threats.