Once again, underhood temperature has nothing to do with the radiator when it comes to cooling it. Remember, all the heat the radiator sheds is blown right into the engine bay, so changing the size of the radiator will have no impact on how much heat is dumped into the engine bay.

I don't know what valve cover you're using, but you can't transfer heat any faster then the stock cover does. It's under 1/8' already, and aluminum sheds heat fast. The only thing that will keep the cover cooler would be an oil cooler, and there is absolutely no good reason to have one of those on a stock or lightly modified B18C.

There are three major things you have to worry about when it comes to temperature. Here, in order of importance:
1) Engine coolant temperature, regulated by the thermostat and radiator.
2) Engine oil temperature, regulated by the engine coolant temperature and an oil cooler/thermostat if you have one (Hondas don't, stock)
3) Intake temperature, which depends only on where the filter is located and if it and the intake tube is insulated.

*For turbo cars, there are a couple other points (like intercoolers) that I won't get in to.

Underhood temp is a not important.