I know several people who have 'double glazed' the rear bulkhead window but IMO it doesn't have that great an effect on cabin noise levels.
You should however concentrate on 'lagging' the recesses behind the seats with a good quality fire retardant material & I have found that double skinning the engine access panel & filling with the same material can make a difference to heat transfer into the cabin. I have also used sound deadening adhesive pads which stop the aluminium panels from 'drumming'.
my car has a double glazed window, dont know how it compares to a single, but you can easily have a conversation in the car at track day speeds!!
dave b
