I guess once we have ADR then nodes will increase their SF when they are out of reach of a gateway (even at maximum power), once they are able to reach a gateway they will use a "safe" SF (which might be 1 SF above the minimal SF they need to reach the gateway). So the nodes that have coverage issues at a lower SF might need to increase their SF (maybe to SF12) to get coverage. I.e. the maximum coverage of a gateway is probably at SF12, although like mentioned SF11 also gets a quite decent coverage.
But indeed it is interesting to know the coverage of gateways at lower SF's, as the SF information is available in the API, it can be stored in a database that does the coverage mapping, and you will be able to generate maps for coverage of each at each spreading factor. This would however take a lot more "wardriving" to determine. So at the moment I think wardriving at SF11 or SF12 is best for mapping maximum coverage of gateways.
BTW: a node that sends at SF12 is also quite normal, I guess many nodes are set by default to SF7 as that takes less airtime, less energy and works fine if you are experimenting with a gateway in your surroundings.