I had a couple thoughts that I wanted to share w/ you [regarding] the issue of concealed carry and grappling. The late Paul Gomez was a friend of mine and we discussed this at some length on a couple of occasions. He was working w/ Craig Douglas ("SouthNarc") at the time teaching the ECQC course. Paul said that, while grappling with a student, if he could get his hand on a handgun in a Kydex holster he could take it. He felt that the most secure holsters were leather holsters worn IWB and said something like "They're the toughest to get. If a guy even halfway knows what he's doing, you're not taking it from something like a MIlt Sparks Summer Special or a Kramer holster".
More recently, in a discussion on TPI [Total Participant Involvement training], Craig wrote of the Safariland 537 holster (link below) "I like it. No issues at all gun grappling. Not the most concealable holster but for wintertime or a looser fit jacket it'll be just fine. It's certainly the MOST concealable holster I've personally found with a worthwhile retention device."
[Regarding] Fobus. Their holsters are garbage. I've seen them come apart in gun grappling exercises. This IS NOT typical of Kydex holsters many of which are well made/ solidly constructed (e.g., BladeTech, Raven, Spencer Keepers). I don't think Fobus holsters are actually even Kydex.
The issue [with regard] to retention of Kydex vs. leather is the way that Kydex releases all at once. The same thing that makes Kydex slightly faster (consistently .25 seconds or more in tests friends and I have done), to draw from makes it less secure. It's a trade off, speed vs. security.
If I were a cop who went "hands on" w/ bad guys I'd absolutely wear leather (some high percentage of cops are killed w/ their own guns and I'd hazard a guess most lost them tussling w/ a suspect). If I was taking a class @ Rogers Shooting School, I'd choose Kydex. For most armed citizens it doesn't much matter as both needing the .25 second or losing their weapon gun grappling are extremely unlikely and one would never know which one of these unlikely events would occur. I wear leather because I find it more comfortable to wear all day.