I imagine that put upside down it would reduce drag. My trips are local and I mount it topside for two primary reasons.
1. I have mounted two additional grip handles; one just in front of the rear hatch and the other just behind the rear seat so I could lift it by myself to the Hullavator cradle when soloing and it works like a charm. You can see the two handles is a couple od the pictures, particularly where the Oasis is on it's side on the cradle.
2. I usually launch within 10 minutes or less from my house, I launch right side up and it gives me access to the additional grip handles to unload, otherwise without a second person it would be almost impossible to handle because of the wide spacing of the softgrip handles. That being said, if I were to take a trip to see family in Virginia Beach, I would mount it upside down with help for the aerodynamic benefits. REgarding centering, the Hullavator system cradle is to the side to allow extension beyond the car to avoid contact/damage to the side of the vehicle.
In regard to ballast: I found my perfect solution, and this kayak is a battleship, and once the ballast was dialed in , it became a very capable and stable kayak. First of all I'm 270lbs. and without the ballast, the bow would be high out of the water and it wouldn't track properly so... the solution: Is the Straight Line Sumo Seat Sac 125lbs wakeboard waterbag http://www.wakemakers.com/straight-line ... 25.html#sf
. I partially fill the bag, perhaps half full, and place it far forward of the front seat and use the Oasis's bungee to secure it and the bow sits very level and it cuts through the swells and tracks very well.
Initially I wasn't happy with the Oasis due to my initial impression that as a tandem it would be impractical to solo in due to the weight, spacing of the handles and the ballast issue. Now that I have the solution to all of these issues, I feel that it is one of the more capable Hobie Mirage models.
I hope my solutions may be useful to others that are considering the Oasis. It is really a great kayak.