My current set of snow tires have 255 rears, however I cannot compare to the 225 because I haven't used those yet. I have read of a few people who run regular tires that are 225 front and rear, so I don't think the narrower tire should be an issue at highway speed.
I have never had an issue with the 255's but I would like to try something narrower next. Obviously the Boxster is not a 4x4, so driving on snow takes some effort, but I have never had any major issues.
