I have never used an AMK bivy, but I use Apex a lot here in GA where humidity is the main killer. I've been able to go to sleep in wet clothes and wake up dry, and he quilt is dry too. I really like the Apex insulation and have taken the 5oz Apex down to below freezing with a Alpine Light jacket.
As for you suggested plan, my main concern is actually with the polycryo. I have heard that it is very difficult to sew, and I am not sure if the AMK bivy will be much easier. I haven't personally tried sewing with either materials, but others at BPL have used the polycryo and reported difficulty.