I encourage the devs to read the FAQs from AAC, Surefire, and Gemtech (the three largest manufacturers of suppressors for MIL, LE, and US Civilian shooting communities) about the effects of suppressors. They could also consult the Wikipedia article. I also suggest viewing Magpul: Art of the Precision Rifle and listen to the discussions on the topic of ballistics and suppressors.
While it is true that a suppressor may be used in tandem with subsonic ammunition which IS lower in velocity (and resultantly 'damage'); the practice is highly uncommon in the military due to that very reason. Suppressors are used with normal ammunition.
It should also be noted that the suppressors depicted in game for 6.5mm are far too long. They are nearly twice the length of any modern 7.62x51mm suppressor.