Okay, quick update on my 2nd practice round. Still using the .65/.22 13mm Cobetter double filters.
Took my time and was super careful this round. Also tried an air purge. Filtered 1 ml of bac water. After the air purge, losses were neglectable. 2 units lost out of 100 filtered. Just to make sure I didn't damage the filter after the air purge I filtered a bit more water and there was no difference in the amount of resistance. So I don't think I damaged the filter with the air purge.
I'll continue to experiment and report back (and others please do the same), but based on this, it seems losses for the 13mm filters might be a bit overstated. At least for the Cobetter filters.
One important piece of advice, before doing an air purge, make sure the vial/cartridge you are filtering into has a vacuum. I created a vacuum in the vial I was using by removing air with an insilin syringe, pulling out 3ml of air. Trying to air purge without vaccum was how I lost .5ml of the 3ml filtered last time.