Get the axles either by waiting or being pushy now. We had 4400 lb Lippert axles on a similar size trailer, but never got adequate braking. Both Dexter and Lippert make these 4400 lb. axles but use 10” drums. 5200 lb. axles will come with 12” drums and should provide better braking. I got mine changed to 5200 but the pads are not set in yet. Being pushy would be as simple as calling Heartland customer service and asking for the product manage for Sundance XLT. Then tell him that this has already hit the web and you would like to report a successful outcome. Tell him that they already have 5200 lb. axles in the factory for larger units and that it would be easy for them to change them out before shipment. It is literally 8 bolt/nut sets and two electrical splices. If they baulk on the fan, go find a dealer that can install a Maxxair fan—they’re better anyway.
As a side note, because I was unhappy with the 4400 lb. axles on my Forest River product, the factory changed to 5200 lb. axles totally free—all I had to do was get the trailer to them in northern IN. Maybe you should mention that to the product manager.