Are modern life jackets (foam ones particularly) fairly non constrictive as per kayaking technique requirements?
Yes. But most are Type III, i.e. not really designed to keep your head out of the water if you are unconscious or very tired (only Type I are). However this is what sea kayakers wear here in on the West Coast of Canada, even on remote solo trips, with off shore sections, where the sea can be rough and rescue many hours away.
is storage on pfd as important
The more pockets on your PFD, the better, because those are the ones you gonna be able to access easily. Plan for the worst: you are floating away and your kayak is gone. What are you going to need?
I carry and have tethered on my high volume PFD: VHF radio, signaling mirror, GPS, knife. I have also a quick-release tow system built in. I have a Waterproof headlamp in one of my spray jacket upper arm pockets. And a survival kit in a banana back kept in my back (no, it doesn't prevent me to roll).