I wouldn't go to IST for BCN, not for 7 hours/backtracking
Unless
- you're flying in J
- you really want to see the lounge
- there's no other award availability (but you are buying cash?)
FYI I've tried TK J (on 777 & A333) and IST lounge (twice), myself (Transit in Y) and with family (in J) for a week in Turkey