With a properly sized and efficient exhaust system, I'd bet cutouts get you LOUD, and that's it. But frankly, I've always raced corked up, so I'd pass on extra noise for no reason.
This. As is often said, an engine is an air pump. It's a complete system and there are a lot of things that work together to tune it to specific conditions. Needing "back pressure" is one of those myths that always annoys me as it's focusing on the wrong thing. Back pressure is not what you need, though it's often the easier thing to measure. What you are really after is flow velocity to help pull the gas out of the pipes. Back pressure is often just the side effect you see from that because a narrower pipe will flow faster, but at slightly more restriction. The other thing to be aware of is where that restriction is in the system. You could just crimp the end of your tail pipe if you think you need back pressure and it would only hurt you, but stepping down the whole system from 3" to 2.5" could net the same change in pressure, but better overall flow in some cases.
Though to the point of the original post, I doubt open headers will make much difference at all if you have a good exhaust system on the car to begin with. I have some electric cutouts on my car for pure novelty factor more than anything. They are fully variable so I can open them as much or as little as I want, so I just call them the volume switch for the exhaust. I love the way they sound fully open while idling, but I do think they are a bit trashy sounding to drive around town with open. If I crack them just a bit it's not too bad, but they stay closed more often than not anymore. I've debated hooking them up to the Raspberry Pi and Megasquirt setup to automatically control them with rpm, but they don't move near fast enough and it would likely wear the motors out as I'm pretty sure they aren't meant to be used that much.