Gavin,
just a couple of points about the decompression plate -
the plate needs to be a tidy fit on the block dowels to stop it moving around, the plate needs to behave like an extension of the block if the headgasket is going to have a chance of working properly.

Think very carefully about fitting two gaskets - unless you get very lucky and/or aren't using much boost, it's only a matter of time before one of the gaskets fails.. IMO the plate and two gasket assembly has too much 'give' and the potential to let the plate move around too much, and the gasket will eventually fail. Speak to Matt-SAB for example, he had that arrangement on his S1 with a Madforit plate, and replaced it with a single gasket after it blew..

I don't think it's a great idea to have the plate/gasket larger than the bore either, as you can imagine, this leaves a cavity round the cylinder, between the block and head, which will create a relatively cold 'trap' for fuel/air mixture, upsetting the combustion process and emmissions due to it not burning properly.