My guess is that going from a 55F depression to something greater will not impact the drying time a lot.
If you are in the inland empire region, the fir might be easier to dry, but for the coastal white fir if your final target MC is around 15%, that is a very large depression. Seems like it would contribute toward overdry and MC variability. I'd be inclined to have a big depression in the middle (like 40F) and make it smaller at the end (like 25-30).
A little higher wet-bulb may make the kiln a little more energy efficient too.