i’ll just bring my 2 cents :
if there is no synchronisation between 2 BTS, it will not degrade the KPI of the network. Indeed, it is actually a normal situation ! 2 BTS are usually NOT synchronized. It is the MS that will re-synchronize automatically every time it is performing a HO to another BTS.
Now, between BTS and BSC and MSC, there is always a synchronization : they can communicate on their interfaces.
A global synchronization between all network elements is not necessary for normal utilization (voice calls, GPRS, EDGE, SMS, etc..)