I just got my ThirtyTwo Prions today. I'll now confirm for you that the Level 2 liner does not have those nasty looking plastic things on them. The whole thing is foam. It does have stitching in the shape of a circle in the same spot on the outside though, and it's just a little thicker on both sides of the bottom to cushion the Achilles tendon of the foot. My ankle bone sticks out close to where yours does due to being 6' 4", and the Level 2 feels nice.
Those plastic supports on the higher-level liners are designed to really squeeze your heel to minimize heel lift. They look like they'd be pretty uncomfortable for me too.
I would give the STW Boa a try, or any ThirtyTwo boot with the Level 2 liners before considering another brand. The quality of these boots, now that I have them in person, is simply the best I've ever seen from snowboard boots.
Or you could try those and try another brand at the same time. I'll update my thread later if I have time.