Your fine. Whoever shot you down is just jealous that your getting the same performance as him, and spent half the amount of cash that he spent on his intake system.
Testing on this site has shown that underhood temps are only about 10 degrees warmer than outside ambient temps, when your speed is above about 20mph.
While sitting, the temps do get warmer, but goes away as soon as you get moving again.
Something else you have to consider, is that the intake air temp can reach temps of almost 400 degrees upon leaving the turbo. Especially if your running above 25psi (outside the efficiency MAP for the stock turbo).
On the superduties, the Charge Air Cooler is used to help bring intake temps back down, but they are still measured in hundreds of degrees. This is part of the reason for Water Injection. The heat in the charge air turns the water to steam. This lowers the air temp, which in turn does help lower combustion temps.
If your still worried about underhood heat. Its not hard to fab up a piece of aluminum to make your own shield, and put some weatherstripping along the top to seal to the hood.
One thing you can be happy about, is that your paper element, disposable tymar filter element is a FAR superior filtering media. It might not flow quite as much as a washable filter, but it for sure filters BETTER. And since a diesel can move as much as 5 times more air than a comparable gasser engine, its imperitive to have the cleanest air you can get. Washable filters are notorious for small holes opening up in the filter media after just several washes.