any one know how much less noise there would be with 20" vs. 22" tires?
It mostly depends on the tire tread, not the size.
Agreed @darlin. My MX has 22" Onyx rims with Goodyear tires. Almost no road noise.
I understand additional cabin protection from tire noise can be obtained by lining the wheel wells with advanced sound proofing materials.
I believe most tire noise comes through a mechanical path the suspension into the body.
The 20" Conti CrossContact LX Sport have a band inside the tire to quiet the ride. See the "Continental Tires on Model X" thread. I have these, they are terrible. Gone by week's end: unrepairable and 2/3 worn-out after 4100 miles.
Because the 22" tires are stiffer, I believe they will be both noisier and give significantly less life than the 20".