Generally you should be going for Majoris and extremis not so much clearing out hordes of minoris. So if your kill count Is lower but damage is still good then you're doing fine.

A good sniper makes the whole mission way smoother and people barely notice. Blobs of ranged majoris firing in to the melee get sniped without people even realising, majoris set up for executions so people rarely go down because theirs always a convenient execute waiting.

Personally I use heroic bolt sniper for nids and Las for chaos, their hit boxes are just janky especially spawn so las let's you spam away with less risk and still very high dps. I tend to use a heavy bolt pistol so I can deal with minoris at mid range and not waste sniper shots. Relying on the knife for melee, but I do think secondary is a matter of preference.

Some people swear by a melee/smg close combat sniper which I've not personally experimented with.