I think that the Islamic World if reunified by the Fatimids around 1000ish will be spared the Turkish invasions; but will it stagnate after that? I'm not sure. I can see the early eleventh century following Basil II's death being a major headache for the Romans, and I think that the Macedonian Dynasty will be overthrown shortly after the death of Constantine VIII, in favour of a more millitarily minded line of Emperors who will attempt to hold out against this new Caliphate. Perhaps George Maniakes will gain and hold the throne ITTL?
The new Caliphate and the Roman Empire will be far more evenly matched in the eleventh century than they were in the eighth. Here, Constantinople has just wiped out its Bulgarian nemesis, has stablised the situation in Italy, and is booming agriculturally, in stark contrast to the exhausted and divided state the Caliphs conquered in the seventh and eighth centuries. I think that Syria and northern Iraq will become the scene for a wearying series of on-off campaigns for decades, until either Byzantium or the Fatimids finally collapse.