What is the proper serving temperature for Turkish coffee?
The temperature at the time of serving doesn't matter. 

The Ottomans knew how to enjoy life. Coffee was served on social occasions, which happened at a slow pace in that part of the world. The ottoman effendi would spend hours sitting there, smoking a nargileh, endlessly turning the beads of a rosary, and taking a sip of coffee now and then. Even though it is served in tiny cups (which don't hold as many sips as your typical US mug), it would cool down considerably until it was drunk. 

Note that hot food is a cultural tradition typical of Western cuisine. Being hot is not a quality attribute of food in Turkish cuisine in general, many dishes are eaten at room temperature. Coffee is still preferred to be hot, but this requirement is viewed with a much more relaxed attitude than in the West. If the coffee has gotten cold, it is still considered good enough to be drunk to the end. Or rather to the sludge, you don't drink Turkish coffee to the end.