I worked down a 225-55 NOE in a 223 with 1-8 twist to sub sonic . It's right around 3 gr of Unique . It's not much louder than the hammer fall of the Axis I was shooting it in . I really wanted a 225-80 but it hasn't been possible yet . I have a 37gr mould but it is like 2100 fps on 4gr of Unique in 223 .

Alloy will be key to impact performance for a 22 cal sub or even close to subsonic bullet a nice 50/50 WW/pure or 1-20 alloy water dropped should be plenty to shoot good groups and still open up on imapct .
you might be able to go as soft even just the above air cooled . With a check of course .
I've read that Bullseye will make the hammer fall the loudest part of the same loads .
No suppressor here just low to no muzzle pressure .
Altitude plays a big roll in when a load is super or sub also I used to shoot a lot of 1150 fps stuff that was good to go at 4100' MSL but here at 300' it doesn't shoot for beans .