If you're looking to speed up your levelling, I found that the Overkill/Deathwish stealth jobs with people who know what they're doing (which, if they are playing those, they usually are) are simply unbeatable. Not only is the risk bonus around 13 times bigger than the actual mission XP you get, you get +5% XP for the next mission, and a successful stealth job is relatively far more simple than a successful loud one. Although I *still* cannot try to get into the Go Bank vault without tripping the damn motion sensors.
Also: AK 7.62 with a Bigger is Better silencer and Ghost bonuses to silenced weapons is pretty damn ridiculous - you get an assault rifle that can 1HK FBI Maximum Force responders if it doesn't bounce off the armor, it has fifty-something damage with all the range of an AR.
ECM rushing DW stealth maps has always (well, since the exp rebalance) been one of the best EXP earners. 4 people with maxed out ECMs can finish almost all the stealth maps in the ~5 minutes of free "stealth" you get (including FF D3 though it's very tight). Actually stealthing the "correct" way is usually terrible exp because of how finicky guard pathing is and the horrible desync issues which make teammates set off the alarm even though it's not their fault. Running overkill hox breakout/HLM is usually much faster if you don't have a dedicated ECM rush team.
Now that everyone gets the headshot perk, 50 damage is not a breakpoint - you're effectively doing the same damage as someone who's got a 40 damage gun. Max. force responders (tans/browns) need 80 damage to kill in a single headshot, not 50. For this reason, the AK, eagle heavy, falcon, and probably other guns are just utterly garbage worthless tier since you're doing the same effective damage but have ~half the ammo pool (and lower ammo pool = lower pickups so it's even worse). Makes me sad because I absolutely love the eagle heavy but I can't justify bringing half the ammo (and a smaller mag) for absolutely no upside.
Ghost silencer skills are garbage tier. There's no reason to ever get them, they are a total waste of points. The only situation they are ever useful is silenced LMGs (which is a stupid concept) and even then it's not worth the huge point investment. I really hate to say this, because I hate the idea of telling people what to pick and what not to pick for a co-op game but they are really, really bad.
Also: Bronco revolver is the best sidearm, from what I've seen. Rarely bounces off the armour, and often 1HK as wells.
Bronco is ok because it can make 80 damage to 1-shot tans and still has... decentish concealment (I hate not having a laser sight though). However, tan armor does not work that way. If you don't have any armor pen talents it will block 100% of shots from all guns, armor penetration (and snipers) are the only thing that can defeat it, and the penetrate chance is the same for all guns.
Edit: 80 damage with perk, not 100 to 1-hit kill tans.