As I noted above, I can't come to any conclusion as the effectiveness of the cooler. There is no way to know what the actual performance of the cooler is under identical conditions in the same car as it's just not feasible to switch from the stock to aftermarket cooler in the same vehicle in the amount of time needed to get an accurate assessment even if you had the necessary aftermarket gauges.
In regard to the stock temperature gauge, I'm willing to bet that BMW can/has engineered the gauge to place the needle on the gauge wherever they want, and it's very possible that a given needle position could represent a temperature range perhaps plus or minus 20F or more. Unfortunately the days of having accurate (i.e. indicating the "actual" temp plus or minus some small error are long gone) gauge from an OEM particularly for oil or water temp aren't going to happen anymore due to the average user getting concerned or freaking out about a given needle position. Consequently, you'll need a different aftermarket gauge set up to show the effectiveness and then you're still stuck with trying to compare the same car or like cars under the same conditions, a no win situation.