I would think that would be adaquet. The cool down usually discussed is to cool the turbo charger. When under a hard pull and the turbo working, the bearing section of the turbo gets very hot. Oil cools and lubricates the bearings. If you shut the engine down when still very hot, the oil can "coke" and block oil flow resulting in turbo bearing failure.
In our business we routinely load test diesel generators at 100% load, at that load level the turbo and exhuast can glow red from heat. When the engines are unloaded, the red glow goes away almost immediately and by observing the pyrometer we see exhaust temperatures drop from the 900 degree ranges down to 300 or so in less than 3 minutes. Keep in mind these engines are still running at 1800 rpm so one running at idle speeds of around 800, it would cool quicker. If it would make you feel better or safer, by all means run it another couple of minutes.