finished this trip and CX said they could only check thru to IST. Decided to just leave it and recheck bags at HKG since I had time there. In hindsight it may have been better to take them up on that offer and recheck at IST, since I was going into the city anyway. Plus the J check-in/bag drop line at HKG still took an hr whereas there's an entire section for J check-in at IST and it was empty when I passed by.

Thanks paul00 for the tip on the refund of the HK APDT!