Correct me if I am wrong but since NMWU has a 60 degree rating, then #6NMWU is only good for 55Amps unless it is part of a residential service? (T.2 of the OESC) I am guessing that a sub panel to the garage would not fall under this classification therefore limiting my amperage rating to 55amps and not 60?

I think that as long as the Garage is a residential garage and part of a single residential unit, then 60 amps would apply. A residential Garage is not a continuous load and residential services are non continuous loads.
A duplex with a single joint use garage might not qualify.

Talk to the local authority but I think we would allow the greater ampacity as long as the Volt drop does not get rediculous.

Isn't it a 55 amp feeder fused at 60 amps per Table 13? The differences are that if the load in the garage is greater than 55 amps, the wire is too small; and, the location of the panel in the garage isn't restricted like a residential service.

good luck finding a 55 amp breaker

Twh
Yes you make the most important point. As long as the calculated load is less than 55 amps then the #6 is good and table 13 would allow a 60 amp breaker.