He also said that vanes provide benefits in rectangular ducts with abrupt changes in direction like 90 degree turns to "smooth" out the air in the direction you want it to go. He said for curved elbows the air is already being gently redirected and vanes won't buy you much more benefit.
Yorks drop off after .6". So if you have a .4" supply and .5" return, and then decrease the return, you see an increase in the supply.
As an other example, WaterFurnace rated CFMs drop off after 0.5" ESP (through 4 tons) and 0.75" (5 & 6 ton).
Given how high key1's ESP is, and thus the reasonable guess that the variable-speed blower is going 'all out' trying to maintain 350/cfm but not achieving it, I believe key1's going to notice a good amount of operating cost reduction once he gets his ESP at 0.5" or less. My guess is key1 is currently not getting much, if any, of the cost savings benefit that variable-speed blowers are advertised with.