There are a wide range of factors that impact battery degradation. One of these is time, so the fact the car is 6.5 years old is as significant than the mileage you see on the odometer. Also, it's not just odometer miles, but as explained to me by some I know that develop battery packs, it's more what they call energy throughput, which is the total energy that has gone both in and out of the battery. So 40k miles driving a 200 Wh/mi versus someone driving 40k miles at 400 Wh/mi are also going to see different results.
I think you're also mixing battery degradation with what your energy consumption is per mile. Based upon my June 2016 built MS90D, with 19" wheels, I need to average about 274 Wh/mi to get 1 actual odometer mile per 1 rated miles as displayed by the car. Not sure what you averaged on your 200 mile drive that you referenced. If it's higher than about 270-275, then you're going to see less usable range that the displayed number of rated miles.
I've posted this information in several other threads over the past couple months, but below is plot for my MS90D relative to apparent battery degradation over time. It would suggest that my car over past couple years has started to level out around about 268-270 rated miles, so about 8.5-9% apparent degradation.
How much of what you're seeing is function of cell balance or even the BMS calibration having drifted is difficult to assess. I try to operate my car in a way that I allow the BMS to naturally see a range of 50-80% versus topping up every day. I also tend to periodically take longer range trips which tend to cycle the battery from normal operation over a range of say 20-80% or 10-90% also helping to keep my BMS calibration from drifting too far out of whack. As a result I've never had my range "recover" any practical amount for intentionally trying to balance the pack.
Does 255 rated miles feel a little on the lower side? Yeah, maybe a little based upon my 1-off datapoint. Only you can judge if overall this car meets your needs versus you continue to look for another car.
Whichever way you decide, hope you find something that you enjoy and works well for you.
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