if you look at my pics, I could have possibly put the hole in my airbox further back and routed it to the back of the engine if it were set up like yours (I am assuming a lot with that statement). I personally am seeing intake temps between 130 and 150 (I may have said that already). I could easily get them higher with a better heat shield. I chose not to because I have seen temps approaching 180 (my highest so far has been 179) and I don't feel comfortable with temps any higher than that.
also, think about this. if you have a heat source and you blow a fan across it, the heat source has to be significantly hotter than the expected air temperature that you are hoping to get.
I think you will get hotter temps with your setup but the question is how much hotter. I just don't want you to go through a lot of trouble to see 20 degrees above ambient temp. that would be disappointing. I personally don't like doing things twice. there again, you do have a delema with the header location.
Be the change you wish to see in the world
The wife has an 06.5 Kia Optima 2.4 liter W/Auto. Recently I removed the cold air intake scoop just to see what difference it would make.
With removing the intake scoop air is now drawn in about 6" from behind the radiator instead of in front of it.
While the past 5,000 miles or so have shown a 1.5+ MPG increase I've yet to do any temp checks as to how much temp difference there is from outside to under the hood.
As I have time I'll be putting a guage in the air box to see what temp difference there is with the scoop on and off. It'll be simple to do as the scoop takes less than 5 min. to install and/or remove.
My next step is to make a scoop that pulls air directly off the radiator.
Air right behind the radiator
Since the coolant passing through it is regulated to around 180 degrees, the air coming out is always at a certain temp and there is no chance of it exceeding (lets say) 220 degrees... so it is much more controllable and better in my opinion. Again the air that the engine will be sucking out of the radiator should be around 180 degrees most of the time.
The coolant is regulated to around 180 degrees, but that doesn't mean that the air that moves past the heat exchanger is regulated to that temperature. It depends on how much heat is transferred to the air. If your thermostat is 100% closed then the air temperature does not equal the coolant temperature - it is very close to ambient. On the other hand, if your thermostat is 100% open then the temperature will approach the coolant temperature. Keep in mind that there must be a temperature difference between the air and coolant in order for any heat transfer to occur.
A more accurate control of intake air temperature would be a dual intake for hot and cold air. It would pull hot air off of the exhaust manifold. The right proportion of hot/cold air would be regulated using electronics , two electric motors, and two butterfly valves to allow a correct ratio of hot/cold air that would achieve a desired temperature.
Perhaps an easier way to control the intake temp using air from the radiator would be to modify an air filter housing that already has it. My Lincoln has a butterfly in the horn to the airfilter that switches air from the grill and the stove around the exhaust header. It is regulated at 133F (book say it is 100F) by actual measurement.
It would only take a few minutes to reroute the aluminum hose from the exhaust to the radiator.
If you had a couple of them from the junkyard you could do some very creative things with them.