I've had my MS60 for over 2 years and have made the trip to NC (via Savannah) 4 times. The 171 mile stretch from St Augustine is definitely doable with some due diligence. Here is my experience:
I always leave St Augustine with a full battery. This means about an hour and 15 minutes of charging after my 120 mile trip from Orlando. This is enough time for a sit down breakfast at IHop and bathroom breaks. I've found that if I average 290 kWh/mi, I usually roll into the Savannah Airport with about 10-15 miles of range remaining. I know this may seem too close for some, but I find it is very predictable so I'm over any range anxiety. Of course, maintaining 290 kWh/mi means different things when you consider weather, load, etc. With no headwind and 90 deg F, this means I can drive 75 mph most of the way, no drafting. If any of those variables works against me, I have to slow down. So far, I've never had to spend a lot of time drafting or driving slower than 65 mph when it get colder than 50 deg or I encounter some headwinds.
I do find that I have to pay more attention to my rate of power consumption vs speed more than I'd like, but I only have to do it for this one stretch, and then usually only the once or twice a year that I make this trip.
Hope this helps.