I remember reading something, somewhere, about OEM cooling underhood. It was pretty clear that venting through the hood was something that was avoided intentionally. There was mention of the path of airflow being through the radiator, around the engine, then out the bottom, which is part of the reason for the air dams....it creates a vacuum to pull air through the radiator and down/out. I've been reading up on the '04-05 Duramax overheating issues (pretty fascinating all the factors in that) and the complexities of underhood airflow, apparently more an issue on newer vehicles, is fairly eyebrow-raising.
Headers tend to add underhood heat, they are thinner metal than manifolds and exhaust tubing, and have far more surface area, so shed more heat. They will make things worse underhood actually. As well, if the PCM isn't reprogrammed after headers, you will end up with periods of running lean(er), which will further drive up underhood temps.
Not knowing how your throttle body is made, part of heating the intake charge up at least a bit, could be to help prevent icing. A lot easier than plumbing coolant through it.
Opinion, but I wouldn't spend a second or a dime changing anything up until I had a good idea what is normal for at least a few GM designs. Particularly if your intake tract is stock. It's going to be hard to improve on stock intake ducting in any significant manner.
Perusing my old files, I see that on a hot restart of 214* coolant temp, IAT was 151*. During ten minutes of driving at 59MPH, unloaded, on relatively flat ground, the lowest IAT recorded was 94*. It actually dipped to 94* then went back up a bit. Referencing maximum temp data for that day in the location I was driving shows 70*, indicating 24* above ambient was about as good as it would get. I would not be surprised at all to see this not being a linear relationship, higher ambients probably lead to even larger differences in IAT.
This being a manual trans, 350 with headers turning ~2100RPM, coolant temp showing 203* for the run, with a 3 row radiator, electric fans that didn't turn on, and nothing but an oil cooler in front of the radiator. No AC condensor, no power steering cooler, no trans cooler. "Stock" intake air routing from core support opening through plastic duct to rubber elbow to throttle body.
IMO that's a combo that isn't generating a whole lot of underhood heat in the first place, but still dumps 24* of heat into the intake charge. Old adage says that cost about 5HP. (1HP for every 5* of intake temp) Air moving into the engine bay at roughly 60MPH, with more space than a big block, is going to move a lot of heat out of the engine compartment. With the same size engine bay, but larger engine, underhood temps are bound to be higher based on the heat generation from more displacement, but also from the more circuitous route the air must make to get out of the engine bay.