They are not restrictive. My bronco raptor is making identical power (actually 3hp more) as my ranger raptor. The ranger raptor has SPD catted, the bronco has stock downpipes.
If it's worth the sound to you, then you can do them. If not, they aren't needed.
I actually tuned a DirtD intake a week or so ago. Did well! I haven't personally had any experience with it. Nathan said he'd send me one to test, so I can test it out if you all would like to see how it compares.
That's a great question.
I like the sound, but once you get moving, the scoop from the intake helps push/route cool air in. So there will be a small rise in intake temps while standing still, but once moving, you're good.
If you keep the lid on the intake then noise isn't an issue. Once you take it off, that's when you get the whoosh sounds. I leave it off, personally. Unless I'm in water, or TONS of dust.
S&B is the most tested in my experience and works great!
It sounds like in your situation it would be an issue choosing an intake. Keep that in mind.
Those do better than stock as well.