...In all reality, would you expect much of an improvement over this using subsonic and/or match grade ammo?
Possibly - depending on your rifle. Do you even know what your actual velocities are with that load/rifle combination?
I think you will see a big difference if you fit a better scope for long range. And yes, 100yds is long range for a 22lr. You can get good groups with it (sans wind), but it helps a lot to have more magnification and a fine cross hair.
My CZ is scoped with a 4-12x AO, and I use it for 100yd shots on ground squirrels quite often with bulk ammo. Made three 1-shot kills in a row at a measured 100yds last time out (when the wind took a break). I've shot many respectable 100yd groups on paper as well.
Trust me - you need some better glass. Upgrade your scope and then try different loads.