My experience with underrating turbines is very different, but it might be also due to the engines used.
I have a velox XL, which has been flying for a few years with a k180 detuned to about 12~13 kg of thrust. This year I took it out to put it into a bigger plane and replaced it with a k120 and the fuel consumption went down VERY noticeably, while the power of the airplane is very comparable (the 180 had a much better low rpm push and spools faster, but that's about it). I suppose the k120 is an overall more efficient engine than the k180, but anyways, this is what I experienced
I went as far as removing one of the 3 tanks and only filling up the second tank up to half this season.