Basically if you don't already have a very good mould, then don't bother, any weight savings will be minimal ( about the 4 pints of beer you had last night ) and you run a real risk of distorting the foam sufficient that you can't fair the outer surface. Far better to follow John Lindahl's LR3 or LR2 build method, it produces a fair product quickly.

Another trick particularly if you plan to use carbon is to put a very light weight glass cloth on the top surface, you can see instantly whether you have added enough resin or sqeegee off excess.

Best of luck