TK doesnt have PE since 2016.

You should not compare Y-seats to C-seats imho, of course the C-seats are more comfy.

What is you option to fly from BOS to IST? I would compare the coach seats of airlines, that fly from BOS to IST.

As far as I know, TK is the only airline flying directly this route.

As *A-carrier you have LH and UA, but you have to transfer somewhere within Europe.

Non *A-Carrier is: AT, QR, KLM, AF - you have to transfer somewhere.