Can a un-used NONCLUSTERED INDEX still enhance query speed?
Yes it could, when SQL Server decides that the statistics from that index is more accurate/useful and uses that stats to do the estimates and come up with a plan.  

I have come across situations when SQL Server has decided to use stats from one index and scan/seek another index.

Edit - This might not be applicable cause I just realized that you have disabled the index.  I have not tested that scenario.