Thanks for all your replies.
Hmm. I haven't used G10 before so I hadn't considered its rigidity and weight as negatives.
Also, as anyone who owns a boat knows there are really no flat surfaces on the deck of a boat so the idea of using Balsa certainly has its appeal. My T27 was originally Balsa cored I believe. I think I need 3/8" Balsa though and 1/2" might be too thick.
The areas I plan to work on are small, I hope no bigger than 1' x 3' per side deck (guess I'll find out how small when I start tearing into the deck).
Follow up question: What supplier do you use to buy Balsa core?
This place sells it: http://www.specializedbalsa.com/products2/end_grain.php
Do you think it is available more locally than ordering it from a place in Colorado?