Incidently, our very own forum moderator Ken Lunde has an extensive holster page show casing several of Matt's holsters, including one for his P99C. Be forewarned though that the price of adminission for one of Matt's holsters is a 18 month waiting list. Well worth it IMHO but others may think different.
If you are looking for maximum concealment, I'd be looking at holster designs with the belt loops as far from the gun as possible.
The Delfatti holster linked above has the loops centered on the gun, which invariably makes the bulge where the gun is bigger.
I would highly recomend the Milt Sparks Versa Max 2 and Watch Six.
I would also like to reccomend you search Tucker Gunleather. He makes excellent Leather and Leather/Kydex hybrids.
You might have to wait a few months, but not 2 years.
As far as the Leather versus Kydex thing goes, it's a toss up. Kydex can and will break if you try to flex it too far, which is why I'm not sold on holsters like the UCR.
Leather can absorb sweat, which can eat at the finish on your gun. Leather, if it gets any kind of sand or dirt in there (once it's in, you'll never get it all out), could easily leave large ugly scratches on your gun when drawing and re-holstering.
I prefer the hybrids like Tucker's "The Answer" which holds the gun in kydex, but the kydex is mounted to much more flexible leather that can't break. My next choice would be all leather before all kydex.
I've been carrying my fullsize P99 in a Tucker "The Answer" IWB for 2 or 3 years now, supported by one of Tucker's belts.
It's very comfortable, stable, and most of all concealable. The belt loops are spread out a couple inched from the slide of the pistol, pulling the gun in close to the body. I've got enough padding on me that my body fat gently molds around the outline of the gun. I carry all the time in casual clothes, or just jeans and T-shirt. If you've got any excess body fat at all (I don't mean you have to be fat) there's no gun outline to print.
+1 on Tucker Gunleather. I'm currently carrying my Sig P229 .40 in a Tucker Answer, and love it. I only have two complaints with it: 1) the holster mouth has no reinforcement, and I really have to struggle to re-holster one-handed; and 2) I wear mine at the most extreme cant possible, which puts the forward belt clip pressing against the slide, instead of in front of it.
I'm seriously going to consider the K&D Holsters "Cochise" for my soon-to-be-had P99c. The Cochise seems to be the solution to problems 1 and 2 that I have with the Tucker rig. Check it out here.