Thanks for the replies people, its good to have many opinions. Still building a consensus however.
The Infiltrator pulls ahead through killspeed on regular mobs. If you're using the Widow you don't have to wait 3 seconds between shots while you wait for AR to refresh, you just pop out and bam get another kill. No Cloak needed, just aiming a headshot is all thats required.
True, my insanity infiltrator run pretty much wiped out all regular mobs so quickly and wondered "what? is that it?" But ammo becomes a problem with higher level mobs I found, takes 5 shots to kill a scion, while it's only 3 shots with a soldier and A-rush. Also Assassination cloak has 2x the cooldown of a-rush making it not so good for using it to maximise damage all the time.
To be honest, I found myself rarely using cloak as a damage booster. It was more of a disengage/reposition button, or to have a little fun by running up and bashing stuff to death.
Hahaha yeah! That's always fun

Yeah its possible, but its a lot harder to do so when enemies are moving laterally. In that situation you have two choices. Take the shot and potentially miss, wasting valuable Widow ammunition, or hunker down and wait for a good clean shot.
Enemies moving laterally was hard to hit at first but I learnt a trick - set your mouse sensitivity to high. After some practice you can hit moving targets quite easily. Most of the problem seems to lie with enemies moving faster then your aim in my experience.
So far it seems overall infiltrators can dish out much more damage per second in a sniper role but not damage per shot. But then comes the ammunition problems. Argh... should attach a scope on the heavy pistol!