Weight aside... you can bend ply wood.  Here is a picture of my SOF where I used 6mm for the coaming, steamed it and bent it.  The cockpit bend was actually done courtesy low pressure weather system that brought high humidity.  This bend is much more tight than the deck C17.  Since it will be impractical to build a steam box to hold that, you can always pour boiling water over the wood as you work it.  It will be tough and the flexibility is as much the type of ply as the thickness.  I have two pieces of 6mm, one Meranti and Okoume, the Okoume is 100% more flexible but more so on it’s with and not it’s length.  Go to the yard and have a look at the ply they are selling, check out its flexibility.

As for the weight, I googled and found that the weight is about 50% greater… kinda makes sence since 4-6 is 50% thicker… big question is how much of the boat’s weight is wood… let’s assume that you end up overall 50% greater, then the 17 goes from a 45lb light weight to a 67.5lb middle weight.  Is that a weight you are prepared to live with?  More importantly, is that a weight you are prepared to hoist up on top of your car every time you want to go for a paddle?  I am really appreciating my 30lb SOF yak every time I go for a solo paddle.  The C18  (once she's done) I think I will save for when there will be a helping hand available.