Aero Mad,
Hang on ... 2 minutes by shuttle bus and 13 minutes by train = a 15 minute journey time? You mean there's a train every minute just in case the aerodrome shuttle bus should be arriving or departing?
And by your calculations all the passengers and going to be coming from/going to Brighton ... That's kind of like suggesting all of Gatwick's passengers come from/go to Crawley!
So what if the passengers are coming from/going to Eastbourne, Redhill, Croydon, Sevenoaks, Winchester and so on???
Your transit times don't look so favorable now do they???