I added a layer of 1 inch neoprene between my carboy and the insulating sleeve, and that seems to help it get lower quicker. I've also cut the toe or if some thick snow socks and I use that to insulate the space between the unit and the lid as that's going to be the coolest part of the rod (being that it's right next to the cooling unit) and therefore the largest risk for wasting cooling power.
I'm also working on a 3D model to create exhaust shrouds to direct the hour air more up and away from the fermenter and the cooling rod. I've got a working system for that now but I want to change the design entirely too a more elegant solution before I share them with y'all.
With those upgrades I've gotten to 40F below ambient, and the system was running around a 50% of the time. It also seems to bring the temp down at a faster rat, but I didn't think to test this