How much stronger is epoxy to polyester resin when using with Carbon/Kevlar and Fibreglass? It appears to be around double the price, but is double the strength?

yes, you could use a product such as our GC50 which is Polyester and thus acts to prevent the moisture absorption issue with Epoxy Resin but as you say, Kayaks are generally not kept in the water between uses and can dry out fully. Notwithstanding this, it would still be good practice to protect it with an exterior coating (either in-mould such as GC50 or post-mould such as Durepox) but most of the kayaks, especially competitive or adventure type will use epoxy for the enhanced strength characteristics.