I think that adding the lift assist would be the best solution. If you plan to use your flippac as a camping rig only, then go for it. However, I like the fact that i can still put the tailgate down for longer objects and make runs to Home Depot. The barn doors will make that a bit more difficult with rigging up tie up points for the doors, but you still lose the tailgate section for length.
The other thing to consider is that if you plan to do a rear swing out for carrying gear, you will have to design it so that the swing arm can lock out further in the open position than the barn door needs to swing. Not a big deal, but it can be if you are adding the swing arm to an existing bumper or buying a bumper that doesn't allow for that.