We have a silenced 308 in a Remington that shoots subsonic ammo in about .5" group at 100 yds. I haven't tried any farther than that because that's not what we use it for. It's used for night hunting hogs. Might give TTI Armory a try. We have several cases of their ammo in subsonic loaded with a 170 gr RN. I've never chronoed any so I don't know the actual speed but it works. I can only assume that the RN bullet is more stable at subsonic.
http://www.ttiarmory.com/