Cool addon STGN.
I believe in real life the MP5SD does not function properly with subsonic rounds. However, when loaded with full power ammo it brings in down to subsonic velocity. Check
HKPRO.com they say state that it brings the velocity down by about 200 fps.
I think you know this but just in case you didn't. Suppressors don't silence more than they change the sound. Suppressors change the sound of gunshots into everyday noises. The MP5SD for example sounds more like a staple or nail gun than a gunshot, which is quiet and not suspicious. I have read reports that M16/M4 suppressors can reduce as much as 30db and make it sound more like a .22LR which is noticeable but on the battlefield very quiet and nonthreatening.
Lastly, Finnish snipers famously used suppressors against the Nazi invasion have an old proverb dating back to the Winter War
"A silencer does not make a soldier silent, but it does make him invisible."
A good suppressor should elminate the visual and sonic cues that could subject the armed professional to effective counterfire from enemy personnel.
Source: Guns & Weapons for Law Enforcement Volume 12 Number 2. - Silenced Sako TRG-42 .338MAG by Al Paulson.
Source: HKPRO.com
I also concur that a suppressor can increase velocity by acting as a barrel extension. Looking forward to your updates.