Look at the No Loss Exhaust article by David Vizard I posted here:
http://www.ntpog.org/articles/
When it comes to exhaust, flow rate for a given pressure drop is everything. For a race vehicle you want to maximize flow in the peak power band, basically where you are operating the motor most of the time.
The rule of thumb is 2.2CFM per HP. So, given a rough figure of 250HP you would want ~ 550CFM of flow at a minimal pressure drop. To achieve that you will need about a 3" pipe or ~ dual 2" pipes and a single 550CFM (or better) muffler) or dual 225's.
So you're right. But how often do you drive your Pilot at 5400 RPM? That's where peak power is made. That's going to change some with the exhaust but it will probably go up in RPM, not down. Generally when you uncork a Honda you can increase power some but it comes at the expense of low end.
Also keep in mind that unless you look at the exhaust as a system you may not gain much.
If you go to a 3" exhaust I can almost guarantee you:
1) you will trade low-end (power below 5400 RPM) for top-end
2) you will find minimal gains by retaining the stock exhaust manifolds and b and merge pipes
Note that the trade in low-end power will directly result in reduced economy in the same RPM band.
You will make more noise and if that's what you're going for you can achieve that. You can also remove the resonator and put in a free-flow muffler to achieve that. Just the muffler will probably make for the deepest tone.
I've done a fair bit of exhaust system design... if you want to achieve this, any good muffler shop can do it for you. If you want to stay with stainless or mandrel bent (it's silly to do anything that's not mandrel bent) you will pay more but probably no more than getting an aftermarket mfr to commit to what would be a relatively small market.
You can also work with someone like kteller.com or vrsexhaust.com to make a "kit", it would be cheap and any shop could put it on... plus they will do any material and any size you want.
I'd like to hear what a Honda SOHC V6 sounds like with a nice exhaust but I'm not sure I want to hear it on my Pilot I drive about 2 hours each day.