Would be interesting to see how a shroud like that works with air flappers reliefs in it. At low road speeds the fan would pull them shut and pull air through the radiator, at high road speeds positive pressure would open them up and let air flow through freely.
I'd lean towards an air flow problem .... the big difference between low road speed and high road speed is that at low speed the fan PULLS air through the radiator, at high speed positive air pressure in front of the radiator PUSHES air through.
With a box shroud at low speed the fan draws a vacuum on the entire box, effectively using most of the radiator (as long as the offset from fan to radiator is enough). At high speed air enters the entire front of the radiator and then must pressurize the box and escape through only the fan opening in the shroud.
A tapered shroud would probably help a lot, it creates a pressure gradient at speed which directs the air toward the fan opening.
All this assumes the water pump is operating as it should of course.
I'm a big fan of the OEM shroud and fan ... does cost you some HP, but extra HP isn't much good if the engine is too hot to use it.
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