another thing to consider ... if you make your layover more than 13 hours (I think that's the magic number) TK will put you up in a hotel somewhere in Istanbul.
I was put in the Grand Yavuz Hotel in the old part of Istanbul. This was walking distance to everything and I just had a great day. Arrived in IST about 0900 and departed IST 0040. Much better than an airport lounge. If you're already waiting 5.5 hrs another 7 or so might not be that much difference, esp if you are out and about, have your own room etc.
TK have a hotel desk. When you clear passports, go through duty free and come out to arrivals area you turn right and walk all the way down to the Starbucks. This is where their hotel desk is located.
They even sent a driver to collect me for my flight. Cost me 0 $$ & I slept, showered, ate at wonderful cafes, saw many sites, shopping done, caught the trams around, great coffee and desert. No airline lounge could complete IMHO.
I will do it this way again in the future.