Marauder’s set is OK, but slightly harder to maximise defences and resistances, and has less synergy with resists reduction so less dps overall.
Not impossible though if the rest of your gear is up to scratch:
I’ve posted a couple of builds on the forums myself, which were much improved by discussion / contributions from other forum members, and you can also find many other DW Pistol builds for different classes in the builds compendium. It would be worth hunting down and reading through those threads to learn more about their pros and cons.
You can find my two build threads here:
Gunslinger Sorcerer - Fire & Chaos
Warborn Gunslinger Commando - The Pistolier
The Warborn build above sounds very much like They Live’s Witchblade build. I also made a Witchblade version of this build which was pretty much identical in terms of gear and devotions to the thread above, the only difference of course being the skill choices in the 2nd mastery. They performed very similarly (after CoF was nerfed anyway, before that the WB version had much higher dps) but I personally prefer the Commando, just because I’m more used to Demo than Occ.
JoV has posted a Pierce Blademaster Gunslinger as an alternative set-up in his Crossbowmaster build. You can also use his gear set-up with a Commando build instead of Warborn for more dps / pierce synergy.
Chthon has posted a few DW Pistol builds:
DW-Pistols Ranged Cadence Aether Battlemage
DW-Pistols Pure Cold ABB Spellbreaker
[Lightning Conjurer] Dual-Wielding Gun Build
There are also a few other Sorc and Pryo builds out there which I didn’t manage to find in my cursory search.
I’ve also tried a Lightning Warder, with almost identical gear and very similar skills/devotions to Chthon’s Conjurer. It was very tanky (13k HP) but lacked a bit in dps and just wasn’t as much fun as Sorc or Commando to me (or DW Melee Trickster which actually played very similarly!).
I also made an Elementalist, which was probably the most tanky Gunslinger I’ve played. He was almost impossible to kill due to Demo’s defensive skills + Wendigo, but his dps was somewhat lacking 
DW Pistol builds are definitely viable, but you are right in saying that the damage output is not on a par with the top Melee / Caster builds. The game simply doesn’t support ranged DW at the moment like it does DW melee, for instance. Hopefully the Inquisitor will change that.
So…you can make it work, but you’ll get better results (fingers crossed :)) when the Inquisitor comes out.