I have a 13 yr old, non-salt wasting CAH daughter.  I would definitely double the dose while your daughter is feeling poorly.  Our endo calls this an intermediate dose.  If she gets worse, you may have to go to a stress dose (we triple for that), but I don't imagine that would happen with a simple ear infection.  As for if they take longer to get over illnesses or being more susceptible, our endo insists CAH is not a factor, but I can tell you my daughter takes at least 1-1/2 times as long to get over illnesses and catches every little thing that comes along.  Maybe it is just an individual thing, but that is our experience.