My research suggested that an open face helmet was a better choice in a car with working air bags.
UK based commenters indicated that most tracks in the UK demanded a closed face helmet for “open cars”. Whether a Boxster is an “open car” or not wasn’t totally clear. Anyway I went with an open face one (my car is a Boxster and I don’t think I will be in the UK with the helmet and my car).
I did buy my helmet online (and it fit the way I wanted) but I can see where trying on first is a good strategy.