Sometimes it kills in 1 shot, sometimes I headshot the guy 3 times and he doesn't die... what gives?
The Marshall also seems to lag a lot compared with the other weapons

Yes and if you shot the Marshall imedieatly after you re-spawned, you will get all of your energy drained out.

meh its a shoddy hit detection - almost as bad as BF2

Update: I tried it with the silencer and it's the worst thing ever; so much for "fire while cloaked" because it still drains your energy

If you fire in cloak instead of manually leaving cloak before firing, the first shot won't count.