The bigger issue is having enough barrel dwell to cycle it reliably.
You are stuck between the sound barrier cracking and reliable function of the bolt when playing the subsonic game.
7.5" are typically the smallest commercial barrels I have seen available. Some have gone shorter, but you are not likely going to have great performance in both subsonic and supersonic without some know-how. 8" to 10.5" is a safe bet with good barrel makers.
If you want maximum function for subs, open up your gas port and run subs exclusively (you will be able to load lighter bullets subsonic this way that still cycle the action). It will beat the hell out of your gun if you run supersonic loads through it without an adjustable gas block set to half function though.