Actually, it is not close at all. The pressure drop between the stagnation point at the front of the car and the underside behind the front axle centerline is very large at speed creating substantial radiator airflow. Trying to recreate the airflow across the radiator on the dyno that even comes close to that achieved at 60mph on-road is impossible with the typical dyno fans. There is no way to simulate the low pressure condition on the back side of the radiator in the shop, and trying to create a higher pressure front side condition is very difficult due to spillage. This is the reason Dinan went to such extremes with their fan setup. The goal is to create a pressure drop across the radiator similar to on-road conditions.
To the question at hand, I think a fine misting of the radiator surface would go a long way to keeping temps in line. The M5 DME system will quickly pull spark advance and enrichen the mixture the second temps start to rise too far (about 1/2 way through the first pull

). Anything to keep radiator temperatures in line will go a long way to help dyno consistency imo.