Get 500 hp

This is pretty much a reply I had given to someone else who asked the same question...

As others have said, you're already "close" to the 500 hp mark. All 2011-2014 SRT8 392s are rated at 470hp (except the Jeep that's 465). The 2015 SRTs are 485hp thanks to exhaust and intake changes.
Everything that surrounds the 6.4/392 is already pretty efficient. Intake, exhaust, etc. So, you will have to "nickel and dime" it with modifications, so to speak. Five hp here, eight hp there to get to your 500 horse mark. In all truth, most feel the 6.4/392 is already at 500 crank hp, judging by some of the rear-wheel chassis dyno numbers (400-420 average).
The most restrictive part of the 6.4/392's set up are the catalytic converters. That's according to the SRT engineers at some of the earlier "roundtable" discussions that used to occur with a number of the forums. It's not to say that they are actually restrictive. But, that everything else is just that much more efficient. Regardless, putting on a set of high flow convertors would probably net you the highest horsepower increase.
Again, with the whole effort of getting a little bit of hp here and there, this could be a list to work with.
*Aftermarket tuner (Diablo Sport with their Trinity and InTune offerings is the main key player here, though there are other offerings now on the market)
*Cold air intake (try to get one that made of a composite, plastic, or carbon fiber material to help reduce heat-soak. Also, try to get one with a larger opening at the filter end that "necks-down" as it approaches the throttle body for even the most minimal of a "venturi-effect" that compresses and speeds up air flow. Lastly, try to get a kit that has, or can be up-graded to a "dry" air filter as they have better filtering capabilities and don't require oiling)
*180-degree (this doesn't make more hp, but helps you not lose hp due to hotter operating temps. With your hand-held tuner, or through a custom tune, you will set the the engine's cooling fans to come on earlier, to match the lower temp of the thermostat)
*True 84/85mm throttle body (not one that still has an 80mm throttle blade and only ported ends)
*High-flow catalytic converters (Magnaflow, and I believe JBA make complete bolt-in pipes assemblies)
*Under-drive crank pulley (Again, it this mod doesn't make more hp, but allows you to retain some of the hp lost by parasitic drag from the engine's accessories)
*Good cat-back exhaust
** Bring all that together with either a mail-order custom tune, or an actual dyno-tune of the car actually on the dyno.

All of that should definitely get you what you want. If it doesn't, you can still add things like full-length headers,with off-road pipes, or high-flow cats, and ported intake manifold before tearing into the engine for cam and cylinder head changes...

Not sure getting to 500 flywheel HP is worth the money you will spend. Cost out what Perfracer recommended above and your into a thousand plus dollars easy with very little gain in terms of HP/TQ. Lot of money spent to get 20-30 additional horsepower.

One last thing HP is not additive. For example, while this mod claims to add 10 HP and that mod claims 15 HP, you don't get 25 HP when you put them together. If lucky you will see a HP increase somewhere between the two numbers. That's why it is important to get a tune that ties all the mods together and helps you get the most out of your mods.

My recommendation to you is to get your car dynoed to establish your baseline HP/TQ. With your baseline in mind decide what your power goals are e.g., I want 500 RWHP and 475 RWTQ. Then talk to some of the folks on the forum who have gone down this path and can help you with other considerations. Finally, talk to a vendor on the forum who will help you with the technical aspects required to achieve your goals.

Chasing HP/TQ gets expensive. If I have learned anything the key is to sit down and think about where you want to go.

Take Care and Good Luck.