I've been busy treading water, so a bit late to the discussion. I work plenty in plywood, built a boat, cabinets etc. Good old exterior ply (not shuttershite) is pretty OK for most garage type jobs or the odd cabinet in Pete's room etc. Birch ply in DBN retails about R200-00 per sheet more expensive in thickness than the local pine ply's. In my book that's worth the money, but, is it worth the application? Marine ply for anything other than a marine application is bonkers. The stuff is heavy being mostly tropical hardwoods and the glue spec is also a weight additive. yes, you have a nice idea use it, otherwise, maak 'n plan. However, if Lumber City offers you a marine ply without the BS1088 stamp, decline the offer, its k@k and just heavy, makes splinters galore en just generally screws up your Saturday.
