We flew to Brisbane from Istanbul last September with Emirates. We had a 30 kg allowance. We took a domestic flight up from Dalaman and had 20 kg allowance on the way out but on the way back it had reduced to 15kg. By luck and by totally confusing the poor check in girl at the domestic terminal we got away with our rather heavy suitcases. We were going to plead ignorance and say as we'd had 20 up we thought we'd get 20 back but as it happened it wasn't necessary.
Turkish airlines also have a generous 8 kg unchecked baggage allowance which was more than Emirates so we juggled a bit between international arrivals and domestic departures. We also managed to fill our pockets rather well!
We flew two weeks ago to Heathrow, firstly on a domestic from Dalaman to Istanbul and then on to Heathrow, doing the return last Sunday. You have to get your passport stamped in Istanbul - the point of entry into Turkey. However, our bags were checked through to Dalaman from Heathrow, as were we, so we needed to go through Customs in Dalaman. That involved us getting off the plane at domestic arrivals and being bussed round to the international arrivals to claim our baggage. Worth bearing in mind if you have someone meeting you!