It's normally called "unladen sag," and not free sag.
Settling only 10 mm seems like a small number to me. My guess is that adding spacer length to reduce the 10 mm number will result in feeling every tiny crack or bump on the roads. With too much preload, the front tire can't drop into a depression because it's already fully extended so hits the other side with a harsh bang. You want the tires to roll into the depression so suspension damping can take up the impact of hitting the other side.
On both of my Honda bikes, the other one is an NT700V, fork compliance was improved by reducing preload and making the fork oil height closer to the top. The increased oil height slows fork compression as it nears full travel but otherwise can't be felt.