From what I have heard Hechthout is of the best plywood materials around and offers probably better strength and stiffness than other materials which are used for Tiki 38s. So I think I would go with 8mm (but I would check with Hanneke before). On the Tiki 21 you could use 1 mm less if you were using Hechthout, this was mentioned in the plans.
And also Olivier, thank you for the link to Joubert Group in France. I am going to look into that as I am building in Belgium and transport cost of marine plywood from France to Belgium will probably be reasonable.