I'm currently running with an XIV Onslaught. Fairly similar to your original fitting, except that I use 3x dual flak up front (that's enough to eat incoming torpedos about 98% of the time!), use needlers instead of railguns (better flux efficiency, higher range, and I don't need 'em for armor stripping), and run with a pair of heavy needlers in the back - the purpose of those guns isn't killing things, it's forcing annoying frigates to take large amounts of hard flux and back off, and the needlers' high burst damage is better at that.
The rest of the configuration is 80 vents, ITU, Augmented Engines, Armored Weapon Mounts, Advanced Turret Gyros, Resistant Flux Conduits (faster venting, and the EMP protection is nice too!), Automated Repair Unit. (For bounty hunting, I typically drop the repair unit in favor of more vents - but it's a life-saver when I go trawling through hostile territory. The Sindrian Diktat in particular loves to throw fleets at me non-stop whenever I enter their system.)
I don't use Aux Thrusters - I've got both maneuverability perks, and it doesn't seem to make much difference. I also don't use Heavy Armor, because it does make a noticeable difference in maneuverability for me.
I did, originally, use maulers in those two medium slots just forward of the torpedo launchers... found the HVDs work better for me. But then again, I leave the Mjolnirs on autofire (occasionally turning that group off if I can't vent but need to conserve flux), and they're great at popping frigates. Destroyers and up get a torpedo or two.
When possible, vent spam to keep flux low. When not possible (i.e. too many incoming missiles and you need to keep the flak running), it's often worth turning off autofire on a group of weapons to conserve flux - and then turn them back on when you decide it's time to unleash your full firepower (such as, say, a torpedo-carrying Vigilance has just come into range and needs to die right now.)