With engine plus heads and basically any type 1 style heads with reduced finnage, it has been our observation that a set of Nickies go a long way to reduce head temperatures, which would be a good thing to keep your seats in their place and the valvetrain together for the longest time possible. This would also provide superior sealing, which under boost, would be a difficult task unless extreme measures are otherwise taken. I've also seen Nickies go a long way to making Scat split ports reliable, so there's a lot to be said for the cooling abilities of Nickies.
For example, I'm doing a 100mm Nickies cylinder for Raby (~10mm thick sealing surface) with 3.6L 911 gas filled gaskets for optimal head sealing. Maybe this would be a good starting point to make your 300+ horsepower beast fully streetable? To keep cost down, you could always use a set of 103s, and as long as you don't pour more than 1 bar, I think it should be more than streetable (using comparables cylinder wise from Porsche).
BTW, i've been looking for someone to do side by side testing between cast iron cylinders and aluminum ones on a big turbo type 4. Drop me a line and let's see if we can work something out.
Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance