If you really wanted to isolate the carb from the manifold you would also need to use nylon or fiber washers under the carb nuts. Probably minimal gains but thats one less spot to transfer heat.

When I had a carb my vapor lock problems were rare even with a stock hood, but it seemed to be underhood temp as opposed to engine temp that got them. I have an old trans temp gauge hanging near the master cylinder. When it reads over 180* is when vapor lock problems seem to start. In a stock engine compartment full of hot air I am not sure you can make much of a difference. I never had room for a spacer so I was never able to try.