TCF,
I remember seeing that chart for the RR250 years ago as well. I thought I remember someone telling me that they at one point asked for a 3 minute cool down just so they would get an honest 2 minutes. After some research they decided that 3 minutes actually was better! That could be just my crappy memory though.
My info on the Arriel came from one of the engineers years ago who is no longer with us. It would be nice to have that type of chart to reference now using the newest modifications. Thats always been one of TM's marketing points over the RR250 was that it was a direct flow engine as opposed to a reverse flow and that was supposed to make it run cooler.
Max