I think Gortat or Zaza Pachulia is absolute ceiling we could expect from Semih. I'd be very surprised if he were ever as good as either of those players, though.
Probably a very solid 3rd string center though. He has a solid chance of being head and shoulders better than guys like Kosta Koufos, Kyrylo Fesenko, and Johan Petro.
In 2010-11, the 26 year old Gortat is having his best season statistically. He is 2.5 years older than Erden and he is in his 4th NBA season. He is playing 15 minutes and averaging 4.1 points and 4.6 rebounds.
Zaza is in his 8th year, is 26, and in 14 minutes is averaging 4.3 points and 3.6 rebounds. His prime seems to have been back when he was 20.
Erden is a 24 year old rookie, is playing the first 20 games of his NBA career and is averaging 13 minutes 4.0 points and 2.5 rebounds. He is also an inch taller than Gortat or Pahulia.
I realize the jury is long from being out on Erden, but you are already calling this his ceiling?