Hi tillstar!
That's interessting! I am planning to have this route in summer,too. But I think that I'll arrive in Istanbul not until July 14th.

Since I also thought about crossing the Turkey/Georgian border, but had to find out that it is closed, I wonder how you are going to make it?
Originally I wanted to fly back from Tiblissi but now decided to take a flight from Van and from there back to Istanbul and Hamburg.
Turkey impossible in August? I don't think so. According to the books, the average altitude in eastern anatolia is between 1000 and 1500 meters thus I am sure that it won't get too hot there.

Regards
Tristan