Why is it people think that you need a metal gasket for turbos? The Nissan OEM gasket will work fine as is up to 15-16psi. If you need better sealing, you can groove the block just outside the fire ring and add sealing wire:
http://www.moparmusclemagazine.com/techarticles/engine/mopp_0108_isky_groove_o_matic/photo_09.html
I've been running an OEM gasket for at least 10 years now @15psi. Lots of track days, no issues (no detonation though). Which brings up another advantage of the OEM gasket. If you detonate with an OEM gasket you will probably just blow the gasket. If you detonate with a metal gasket, you will break ring lands. What is easier to fix?
Or, you can order a copper gasket from Hussey in the thickness and bore you want. $100 delivered (got the one for my race motor in two days).
Yes, it would be great if Cometic, or Cosworth would start making L6 gaskets. Chances are they won't. The problem with the Big bore MLS gasket from Tomei is that it is only 90mm bore. This leaves I nice groove around the combustion chamber for detonation to develop. The Kameari I believe is also available in an 86mm version. Big bucks though.