RF + Silencer forces Scorpion to be a 6 hit kill at all ranges basically for all intents and purposes.
Scorpion without Rapid Fire takes 10-12 bullets to kill, Scorpion WITH Rapid Fire takes 14-18 bullets to kill. This is because the EVO fires so quickly, with the lag present in this game even if you start firing directly on the person that's how many bullets it throws out from start -> death of player + a slight bit of human delay + additional missed bullets due to more recoil.
Furthermore, for the same reason, the TTK is nearly identical using RF and not using RF for the EVO. (On paper RF's is better, but in practice its the same)
Without using Ext Mags using Rapid Fire decreases the average # of kills per clip to 1, which is basically horrendous.
If you were going to use 3x attachments on the EVO the best set up is most likely Extended Mags + Fast Mags + (Long Barrel/Laser Sight/Silencer depending on playstyle).
Extended Mags changes EVO to 3 kills per clip (average) making it the most worthwhile attachment for the gun.