I've read in several threads where the colder air you can take in the better gas mileage performance you will get. However, on the reverse I've also read of taking in warmed air from the engine compartment and getting significantly better gas mileage performance.

Is there a middle ground? Does the preference change at some temperature point?

What do people do and how do they see it changing their performance, either mpg, my primary focus and power/torque?

Bill

Diesels run better when warm. There is no such thing as a true "cold air" intake unless you install a snorkle. In my opinion, an intake system isn't going to do crap for the CRD. Just let it run warm and it'll be happy.

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2005 Stone White KJ Limited CRD 4x4
Off-Road Pkg
Trac-Lok
Emu 790s/948s
Skyjacker H7088s rear/Bilsteins front
15x7 Black Rock Lobos/31x10.50 Firestones
GDE HOT Tune + TCM Tune
Euro TC
CB radio
Thrush Turbo muffler
AEM DryFlow filter

so could you actually block off the grill and run hotter even in the summer to get better mileage?

Run it the way it is, the turbo will heat it up and the inter cooler will cool it down. It takes extremely cold weather to cause any issues. Unless you live in the far North, high Altitude, Alaska, or Canada you should not need to block off the radiator. If you live in one of those areas the people who live there should know all the options if there are any better than a piece of cardboard.
In a perfect situation, most Diesels prefer 60F, enjoy that when it happens.
Some of use use fuel coolers to enhance the lubricity of the diesel and reduce the power loss in hot conditions.
I have found out the fuel heater is useless at any temp over -10F, even with my over kill fuel cooler that is twice the size needed.
If you want to experiment with running a diesel engine hotter, pick one that is bullet proof like an old Mercedes inline engine with a cast iron head. What some have done on those old beasts is run straight Propylene Glycol, boiled out all the water (just keep adding to the cooling system until you need to quit adding more), run a ZERO pressure radiator cap, and rig up a 250F to 275F thermostat. I don't know what engine oil they use, but the fuel economy goes up. Cummins built a test engine out of ceramic that had no cooling system for the Military, have not heard any more on that one. Doing this on an aluminum head engine is risky. GDE discussed running higher temps, but you should talk to them if you want to give that a try.

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2006 Pearl Green CRD
Magnaflow 2 1/2" Cat Back
KJ Extra Leg Room Brackets, Carter Lift Pump, V6 Airbox, ORM
Fuel cooler, Oil Separator, Progard 7
Gauges EGT Boost Trans Temp Oil Pres, Michelin LXT AT2 245 70 R16
7,000# Draw Tight hitch, PML EX Deep Trans Pan
Centrifuge, SunCoast, Transgo, RAM TCM, InMotion Stage 2
Wife's 99 TDI VW Beetle

Great, Thanks, I'll disconnect my Hockey Puck, as I have never seen -10 here in SE MASS in 25 years of living up here. One less thing to worry about.
And no I don't want to mess with high temp engines, playing with Aluminium engines, I'll stick with putting in the oil drain and that's it.

i think more air flow would benefit the CRD if you have a programmer or tune, but colder air like everyone else says wont do anything for you since the turbo charger just heats it right back up again and then sends it to the intercooler to be cooled again

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05 CRD Limited, SEGR Kit, EHM, Selectrack Alpine DVD, 6000k HID headlights, Aux Lift Pump, 2.5" straight pipe, Diablo Extreme Puck, Euro converter
02 SLT Cummins H.O. 6spd manual with southbend 3850dd, fass 150hd, ddp 150hp injectors, arp studs, adrenaline stacked w smarty, traction bars, 6 inch stacks, level kit, bhaf