Honestly, I'm not seeing a huge difference between them and the
Mattock, a non-automatic assault rifle.
Is there any way to get comparable DPS to the slower rifles? Maybe I'm
just bad at sniping with them, but I can't ever seem to land
consecutive headshots with the fast rifles.
You don't. There's a reason why they are semi automatic and not 1 shot. Whenever you land the first shot, you will cause the target to stagger, not only that you'll get a little kick back so there's no way you can consistently land head shots in smaller targets. The whole point of those babies is the fact that they have a built in scope. This frees up your two mod slots for something nicer, such as damage and piercing. Or in the case of single player, the time slowdown and damage mods.
The M-13 Raptor is what I like to use when going semi automatic, it has a huge ammo capacity and a decent clip size. Use this to unload on big targets with no shields (brutes, atlas with shields stripped, etc) and watch them die really really fast. Also, with the infiltrator, you can get off several shots with this before your cloak wears off, getting the sniper rifle bonus for all the shots. Generally though, I still prefer the 1 shot 1 kill rifles such as the M98 Widow for the infiltrator.
One last tip, you can do a 'speed reload' for the 1 shot 1 kill rifles like mantis and m-98 widow by using a skill about half a second after you chamber the round. This way you don't have to wait for the entire pull back motion to complete before you can fire. This works great when teamed up with a spammy engineer as a fully upgraded Mantis is actually quite light (Salarian engineers get an awesome -30% sniper rifle weight to boot). This improves their DPS just a tad to bring them more even with the 3 clip Black Widow.