Constantinopel hayssen die Chrichen Istimboli und die ThÃrcken hayssends Stambol;
The Greeks had no problem with initial consonant clusters but the Turks did, so it is much more likely that the Turks added the initial I to a Greek word starting with ST, just as Spanish and French add initial E before such clusters.
Are you sure about the Turks and the initial consonant clusters? I always thought it depends on the actual cluster structure. Modern Turkish at least has loanwords such as "brokoli", "graten" or the notorious "spor" where the problem is the word-*final* cluster, not the word *initial* one. While Turkic roots usually do not begin with consonant clusters, it appears to be OK in loans.
The situation is a bit difficult because of the Persian and Arabic adstrata in Ottoman Turkish. Both Arabic and Persian definitely do not allow word-initial consonant clusters at all, which led to a lot of words with auxiliary vowels in Turkish. However, these words already had the auxiliary vowels when
Philipp -- Was fÃr Japan ist der Tenno, ist fÃr Frankfurt Brezel-Benno. - BrezelverkÃufer in Frankfurt/Main
