I'm late to the Tantrum party, but when I saw RVators post I had to respond. Great minds must think alike, cause we both used similar color schemes for our Tantrums:

I used a Himax 2015-4100 in a GWS 6.6:1 gear box with a 12X6 prop in my first Tantrum. It flew great unitl I nailed a light pole trying to land in a parking lot. I liked the plane so much I built another one. I used the Himax 2812-0850 outrunner in that one. It was also a good motor for the Tantrum, but not quite as much zip as I had with the gearbox and 4100 setup. Either one will work nicely.