The OEM filter on almost any car you can buy will flow more than the throttle body. There is no RAM Air effect below perhaps 100 mph, and if you are regularly travelling at that speed (and above, because the effect is weak at "only" 100) you need to properly place the intake and be very anal about any change in direction. All deviations, even small ones, from a straight line will reduce and possibly eliminate any RAM effect.
Place the intake with a bell-shaped scoop where your front license plate is. Route it to the throttle body or carb butterfly with gentle turns and if possible, no turns along the way. Move the license plate somewhere else. Now drive faster than 100 and you're golden.
If you're not willing to to the above, forget the idea of any performance improvement with an aftermarket cone filter setup. Unless your goal is to move the intake above the waterline you intend to ford. Don't forget the exhaust while you're at it. Your truck won't go faster, but will wade higher streams.
For those who say that you can use the high pressure area at the base of the windshield (aka the Chevy Cowl Induction system), well, you probably can't. Test with a manometer and see where your pressure gradients fall.
It is a good spot on some cars, but definitely not all, and in some cases is one of the poorest areas to pick up air. Generally speaking the more "dirty" your windshield rake angle is, the more likely it will offer the required pressure. So the newer your vehicle, the less likely it will work, basically.
As for cooler air, although underhood temperatures can be high, generally speaking they only are static enough to matter in stop-and-go traffic, an operating condition where you won't be asking the motor to offer much power. Once you are travelling faster than perhaps 40 MPH the air underhood is being refreshed by cool outside air anyway, and remember the benefit of cooler air is more power at Wide Open Throttle (WOT) and high power operating conditions. At part throttle there is little benefit.
I like an oil-coated cotton filter (like the K&N) but it's generally not a performance improvement. In some situations where the breathing capacity of the motor has been substantially increased, a revision of the flow capacity might be in order. Kits from companies like K&N are often the easiest way to achieve that. Even then, a higher-flowing OEM unit, perhaps one designed for a larger motor, is just as likely to work.