How much insulation in your doors?
I would think that insulation would be key in determining roll-up door performance on a reefer truck. A well insulated door would perform better than a poorly insulated one.
For the reasons Zorry states, roll-up doors are the way to go.
Diane and I were once on a run from New York to Washington state with a DR-unit with barn doors. After loading, we were quickly back to temp. We waited a long time for the DR-unit to get back to temp. The barn doors had to be opened before bumping the dock and the shipper was slow to load. All the precondition the DR-unit did before arrival was wasted and the freight was loaded into a warm truck body. We bumped the dock with the door closed and opened it just seconds before the freight was loaded. We often close the door behind us when securing reefer freight to preserve temperature. It takes no time at all to get back to temp and the customer's freight is better protected.
With that DR-unit, we ran at the same temp along the same route. Just before the delivery, we parked our trucks side by side and I was able to shoot both doors with our infrared thermometer. The exterior temp of both doors was the same anywhere I shot the painted surfaces (top, bottom, middle, corners, etc.). The temp was also the same anywhere I shot the metal surfaces (hinges on the barn door, frame on the roll-up). The roll-up frame exposes more metal to the outsdie but both trucks held temp just fine. We ordered a high quality door with our truck build.
Once in a while, actually just twice in six years, when the set point is -10 Fahrenheit and we are running cross country on hot, humid days, the roll up door will freeze shut on the inside (no frost on the outside is visible). A single football-style shoulder block on the door loosens it up.