Love the video! That should make for some quality fun with your Suburban.

As far as how much boost you can safely run. Your P400 should be very happy at 15. I don't know how efficient your GM center mount turbo is. I am familiar with the side mount GM turbos, and they were terrible at higher boost levels. The drive pressure to boost pressure was around 2:1 and higher. So if you were running 15 psi boost, the drive pressure in your exhaust pre turbo would be 30+ psi. The higher the drive pressure, the higher your EGT's get, and the more risk of head gasket failure.
I'm currently running a Holset turbo on my truck, with a max of 20 psi, but the drive pressure on my turbo is 1:2 to 1:5 depending on what my RPM is, which makes higher boost levels safer. My engine is just the normal 6.5, so its not built nearly as well as your P400. You can see my current engine here. http://www.thedieselpageforums.com/t...ad.php?t=43766
I know several people running in the 25 psi area with their 6.5's without problems, but it get risky at those pressures. I don't know if the P400 has improved head gaskets, but I've read that the heads have been improved, so they don't crack like the old 6.5's did, and of cours the bottom end is bulletproof! Forged crank, main cap girdle, etc...

I'm really looking forward to seeing your Suburban completed.

Matt