To maintain full accuracy and minimal spread with the Rokstedi, you have to wait about a second between shots. You'll find that if you don't go spammity-spamming, your shots will be true to their mark, and you can put people down in a few shots. I personally don't bother with the Muzzle Brake, since the gun's recoil already settles faster than its spread, and I use the standard mag to maintain full accuracy. Throw on a COGA/Greeneye, and it pretty much bridges the gap between ARs and LRs.
FRKN is just a terrible gun, and out-classed by the Gerund which can fire 3-shot bursts with the same if not better accuracy, and with a smaller pause between them.
Rapid fire?? As it stands the gun already has so much spread, that the recoil settles before the spread has time to. In that case, making it fire even quicker... seems pretty pointless to me. Unless you were going to use it for hip-fire... but even then, it grows to shotgun-sized after a few shots.
I find that the Rockstedi with Rapid Fire, silencer, and acog is useful. I don't think the silencer does affect how many bullets it takes on light / medium body types, although heavy's may take in one or 2 more rounds.
Either way, I do agree with you that the FRKN is not a very good gun. It's accurate but you can't hit anyone sprinting.