On a Friday afternoon the traffic are OK at that region. (North of Kowloon Tong / Road to Airport from Kowloon Tong)
Taxi all the way to airport is HKD 250 or so from HKBU, consider you are in a rush, it's definitely worth. waiting in air side is always more secure. approx 34 min according to google, 30 min if the taxi go speeding (quite common)
Note that your time is before "changing shift" for Taxi Driver and they start head to refill the LPG, if you select this option you probably need to book a taxi.
In your case it's pretty embaress, HKBU is in the middle of nowhere for airport express but the saving of 50% seems attractive (Taxi Fee 90 + Taxi-discounted Airprort Express 30). Taxi + Airport express seems still OK even you miss a train (20 min taxi + 12 min wait for train + 13 min on train).
Discount Information:
http://www.mtr.com.hk/eng/airport_ex...ions_taxi.html
Hong Kong Airport has standardized carry-on policy, so if you pass that before, you are OK.
http://www.hongkongairport.com/eng/p...gulations.html
Basically they don't weight you, but dimension only