Well, I have to admit a reversal on my decision to cut my own cabin air filter from furnace filters.
Over the past weekend, I was out in some moderately nasty & cold weather with a home-cut filter in there, and the heating system did not perform to my expectations.
I thought maybe I had made some error in the filter, so the next day, I examined it.. No errors on cutting/installation, no obvious blockages, so I looked at the resistance of the filter material itself. Using admittedly crude testing methods (blowing thru it with a vacuum cleaner output & measuring how far it would lift a piece of paper), the furnace filter material was significantly more restrictive than the used OEM filter.
The furnace filter was 3M filtrete.
Picked an OEM filter up at the dealer ($22 bucks) and it works much better.