Backpressure isn't what's desirable, appropriate flow is.
This is a subtle, but important distinction. A smooth flow is necessary to maximize cylinder scavenging during the exhaust stroke. When the exhaust is properly sized, this happens such that the inertia of the exhaust gasses through the system creates a low pressure area at the exhaust valve about the time that the next exhaust pulse is nearing its completion. The result is that the cylinder evacuates more efficiently, allowing the intake stroke to draw in more fresh air/fuel mix, and ultimately create more power. If you were to restrict the flow of exhaust such that there is greater than ambient pressure at the exhaust valve during this time (IE Backpressure) this would be worse performance, as the engine could not acheive it's potential volumetric efficiency (VE).
An exhaust too large will effectively keep ambient pressure at the exhaust valve all the time.
That said, I would think that the stock size piping from manifold to catalytic converter is probably enough to encourage proper exhaust scavenging, and if you really wanted the dual exhaust, you would probably need a dyno to measure any performance change. Most likely there wouldn't be a significant difference that couldn't be accounted for by the extra weight of the system.
All of this goes out the window when you're dealing with turbocharging. So far as I've been able to model, there's no such thing as too large an exhaust on the outlet of a turbocharger. For one, the turbo itself imparts inertia to the exhaust flow, trying to keep a steady flow despite the pulsed nature of the exhaust as the turbine blades regulate flow, and second the turbo depends on an energy gradient over the turbine, and the easier it is to evacuate the exhaust gas from the outlet of the turbine, the greater that potential energy. The turbocharged system is also much more forgiving of header design as well because of this. Equal length runners are still ideal, but offer minimal gains in a turbocharged setup.
In most normally aspirated engines, the exhaust system will be optimised for an RPM range, where the rate of flow can be managed in order to create that low pressure point at the exhaust valve at the appropriate time. this RPM range is usually placed to coincide with the throttle position (As it modifies the engine VE) and RPM that the designers expect the car to most need the efficiency boost. This tends to be "highway passing" so in the case of the F23, that's probably 4-5k RPM where you will get the most benefit. Of course there are also elements of cost and noise where the exhaust is optimised for.
If you want to move that efficiency band for racing, you probably want to bump it up to 5k-redline. there will be an exhaust size that does that, but most of those changes will need to be on the hot side of the catalytic converter, and not the catback. Of coruse, I'm not a Honda engineer, so I can't say for certain where the exhaust system is tuned for, but I'm absolutely certain that it was tuned.