With cold outdoor temperatures, a slight vacuum in the house is better. Your warm indoor air holds more moisture per volume, so if pressurized you'd have greater potential for in-wall condensation.
HRV performance figure calculations:
Heat Recovery Efficiency
According to
this, 0.5mm is about .020", 25 gauge. Are these sheets expensive? I've never priced aluminum sheet. How many layers do you have there?
From the photos, it appears to be a high quality build! Very nice work. I'm interested to hear your efficiency numbers.