A group of EV owners and clean air advocates are collecting data and publishing results of battery performance over time. Although random surveys are never completely statistically perfect, this data should be helpful to further the electric car industry.
They have finished analysis of the Leaf data. The most interesting conclusion was that in very hot climates, the batteries seem to degrade faster.
They are in the process of compiling Roadster data, and have 109 vehicles reported including mine. Although the list is not large, Roadsters have been on the road longer than Leafs. I’m looking forward to any conclusions they come up when analyzing the data.
If you own a Roadster, they are still taking data here.