I think if the perdes' werent mathematically explained; we cannot know the answer. Only reason we know what Hicaz is what we play today, is by the tradition.
The rest is speculation, like saying it was todays' segah being 2nd degree of Hicaz as well for ottoman music before 19th century. Or that it has changed in somewhere in 19th century. I will share older records with you that in Iran it was similar to us before. So I can conclude by myself that they made reforms. My argument would be stronger. However, its only speculation. I dont believe in this.
Iran comparisions are always orientalist, yet Iran had many reforms facing towards west or old times. There is no repertoire of Iran classical music from even 19th century nor variety of Terkibs like us. Comparisions are not scientific by this way and even funny. I recommend you to stay a bit distant to western scientists regarding Turkish music, because what they say is not usually what they know or understand.
Amongst the huge repertoire of harder, new and more sensitive maqams being understandable in books and or transferred with oral tradition in Turkish music, Hicaz is very simple and easy to transfer to next generations as it is. That is why we are more likely to accept it being because of the Nays' capabilities like [USER=393]@orkun zafer özgelen[/USER] explained.
I searched deeper and found recordings from 1906-1933 from Iran, using Kürdi and Dik Kürdis in Hicaz. Which depicts a much different picture than Feldman I guess. Who says Homayun is the equavalent of Hicaz in Turkish music? I dont think such knowledge exist in any book, its maybe Bayat-e Isfahan(Homayun) or whatever, I dont know this music 
1) [MEDIA=youtube]9aCDy_LTS-o[/MEDIA]
2) [MEDIA=youtube]mA_wSalqqe4[/MEDIA]
3)[MEDIA=youtube]7n8Gjx6PrdI[/MEDIA]