I have seen some "A" cat "trailers" that actually fully encase the boat and they are angled in the front to reduce windage- You can also mount your mast on top of the vehicle with the front of the mast forward to reduce windage from the mast supports on the trailer and keep the mast low. Most aerodynamic rig I ever had was my old 18sq trailer- boat "knocked down" between races so hulls were put upside down on top of narrow, low storage box with bottom flush with trailer frame and filling the space (to keep drag low under trailer) and mast held parallel fairly low over vehicle- all of the boat/trailer except the mast (and mast supports were round tubing) were in the "slipstream" of my truck so little additional aerodynamic drag between my truck w/ cap on the back.
Another "advantage" of supporting your boat on the trailer by the beams is you can eliminate the trailer crossarms/rollers/cradles which typically are NOT in the vehicles slipstream so offer a lot of drag. Our hulls are pretty aerodynamic (although there is a large difference between the aerodynamics of an "A" hull vs a H16 for instance).

Kirt