Hi Paul
I am pretty sure that the extra resin needed to wet out 6 oz carbon would add more weight than you could save by using balsa.
My strips were 4mm, so by the time they were sanded, they were somewhere between 3 and 3.5mm. The hull and deck are several times stiffer than any fibreglass kayak.

I think a core construction gets its STIFFNESS from the thickness of the core. That may not be the same as STRENGTH. For a sea kayak, 5mm is plenty stiff enough and strong enough for most people. For a flat water racer, 4mm is more than enough. 3mm would be plenty stiff enough, but might introduce some construction problems.

I didn't vacuum bag. I just took a bit of care with the layup and made the seat, footrests and rudder quite light as they can add a bit of weight. My target weight was 8 kg and I keep telling myself (and anyone who is prepared to listen) that I could have achieved 8 kg if I had done the layup in warmer weather.

Allan