Expertise smg is good enough for very lightly armored guys. 5mm silenced won't be able to scratch tac vest guy, let alone tin cans. That's the retarded design of silencer, it reduced the average damage by ~20%. Around lv 18, expertise might carry you through enemies DT and probably able to kill 1 target in 3-4 burst.
Not sure about crit, but if you pick ambush (and you should as sniper), it should be good enough if you have decent crit chance.
Alternatively, just play crossbow. They're designed as silenced sniper. After level 14 (crit power) you basically 1-shotting people left and right without any noise. It's kinda boring afterwards tho
But you have W2C ammo readily available.
Use regular ammo for easy targets, JHP ammo for big, lightly armored targets and W2C ammo for heavy armor and tacvests.
A Silenced Rapid 5mm Jaguar burst for 16 AP, so for 64 APs you fire 28 bullets, (almost) emptying your magazine.
(You don't even need to use meds, can just activate sprint and Blitz to gain +15 extra APs with help from Interloper.)
Expertise for a 5mm silenced SMG means the difference between not being able to damage heavy tac-vest enemies, and killing them in one round.
In a worst-case scenario, you are up against a 50 effective DT enemy. That becomes 17 if you use W2C ammo. That is more or less the avg damage of your silenced SMG at lvl 10-12, so you do not do much damage.
With Expertise you effectively add your lvl/bullet on top of this, say you hit with 24 bullets out of 28, that is about 280 damage. This is not too bad at lvl 12.
As you go to lvl 14-16 and aquire better SMGs, you will do enough damage with W2C ammo to reliably take out anyone. For extremely hard enemies take Adrenaline and use a Taser beforehand to stun. Add in Blindsiding +15% damage on top and you are good to go.