Need a couple questions answered first.
Do you know what coolant mix you are running?
Many of the racers can get a drop in engine temps by reducing the antifreeze percentage in the coolant mix from 50/50 to something like 20% antifreeze 80% water, and adding a wetting agent like "water wetter". This increases the heat capacity of the coolant and increases the ability of the cooling system to carry heat away from the engine.
I have also had success going to a lower temperature thermostat. If removing the thermostat is a option, you might want to look at that.
http://www.racerpartswholesale.com/redtech3.htm
Another odd ball problem might be that since your driving at greater than design speed, your over speeding the water pump and making it cavitate. If that is the case, its ability to move water may go to hell in a hand basket at high speed, and slightly smaller drive pulley that underdrives it a bit will get it back within its design rpm range and stop the cavitation.
You can also buy accessory 12V water pumps for racing applications to build a secondary cooling system.
In the WRC cars they also put a water spray system in front of the radiator and under high load spray a mist of water directly on the front of the radiator, this can drop the air temp of the cooling air by > 30 deg F, plus the cooling effect of the water that actually reaches the face of the radiator as it flashes to steam on contact with the hot surface. Place the nozzles so they are as far as practical away from the radiator so the spray has time and space to evaporate and spread out across the surface of the radiator.
These changes would attack the temperature of the engine directly.
You may want to consider an aerodynamic approach as well, try to cut some holes in the hood about 1/3 of the way back from the front edge. This is typically a low pressure area at speed and should help the fans pull more air through the radiator. You could also hang a strip of rubber or similar material under the front bumper as an air dam to reduce the air flow under the vehicle, this is commonly in racing applications and lowers the air pressure under the car at speed and frequently improves cooling.
http://www.mirafiori.com/~danb/spoiler/
http://www.datsuns.com/Tech/aerodyna...nd_cooling.htm
You may also be able to make use of your interior heater system. Route the coolant lines for the interior heater, to an additional core mounted so it acts as a reserve radiatior. In a racing environment you can drop the engine temp significantly by turning on the heater with the blower on full, unfortunately it makes the interior quite hot --- a problem you certainly don't need, so an external mount would seem to make sense.
Given the maint capabilities I suspect you have available (ie your not paying for replacement turbos) the simplest set up to rig would be to put a spray nozzle in the air intake as far as possible infront of the turbo compressor. On the Buick Grand National turbo charged cars they frequently run a 7 or 8 GPH nozzle fed by a 100 psi pump. I have the same setup with a 5 GPH nozzle on my 2 Liter WRX.
FYI the supercharged engines in WWII fighters used this exact system to control detonation for "war emergency power". They sprayed both fuel and water/alcohol mix (50/50 water methanol) directly into the eye of the supercharger. It was ultimately used by all combatants during WWII, the Japanese introduced it late in the war on their A6M zeros, the Germans used it on the FW190 ( BMW 801D-2 engine) and the Allies used it extensively, on nearly all combat supercharged aircraft which were powered by the Pratt & Whitney R2800 engines, and the Spitfire and P-51 which used the RollRoyce Merline engine family.
As long as the spray is in the form of a fine mist it will not cause significant errosion of the compressor blades. I have been doing it for quite some time, and I know of others that have done it for years. If you set the turn on pressure properly the water spray will only come on during WOT conditions when your under high engine load.
The Tractor puller folks also use water injection extensively to control EGT temps on their unlimited tractors. Some of these boys are running 200 psi boost pressures. They typically see a drop of 200 deg F in EGT. The info I've seen is that they inject at about 3:1 ratio fuel:water at 160 -180 psi manifold pressure. On gasoline engines the ratio ranges from 10% - 50% of fuel flow.
You can use exhaust color to help you tune the injection. If you have too much water the exhaust goes white, just right, is supposed to be when the exhaust color takes on a gray color, and if the exhaust is still black you need more water.
Injection pre-turbo compressor, will not only cool your exhaust gas temps but will actually increase the effeciency of the turbocharge compressor in high temps --- ie you get more power !!!!!!. The hotter and dryer the intake air the better this mod works. The cooling due to evaporation in the intake pipe dramatically improves the total air flow through the compressor, and reduces the work required to perform the compression.
For every 11 deg F increase in air temp your engine power potential drops by about 1%.
Modern gas turbine power plants use the same system to increase the power output of gas turbine generators in hot weather. It is commonly called "wet compression" "over spray" or " High fogging". It is a very well developed idea in the power generation field. They typically spray about 3% by weight of the air flow.
http://www.caldwellenergy.com/pdfs/EVAPOR.pdf
http://service.spray.com/Literature_...pplication.pdf
Let me look around and see if I can find you any additional info directly applied to diesels.
[edit] I did find some info that diesels really like a water methanol mix. Like Nitrous, you can get carried away with methanol mixes greater than 50/50. At that point the alcohol seems to act as additional fuel, and pushes cylinder pressures up enough to blow head gaskets.
Larry