One common test is the MAP scores, and the IAPMO may be similar...they are useful in comparison, but not true to real-world results. They don't take into account bowl wash, splashing, and actual turd disposal. The test is done with plastic wapped paste. The real stuff is quite different. On the MAP tests, the originator of the test said anything over 500g is fluff. But, the big thing is the manufacturer's have learned to to maximimize the test results, but they don't always equate to reality.
I don't think I've ever seen a Mirabelle toilet, so I can't say. I do own some Toto toilets, and they are head and shoulders better than what they replaced. one of the things to consider is if they use common, off-the-shelf parts in the tank. If not, it will likely be expensive and time-consuming to repair when the time comes. Toto uses easily attainable stuff except on their whiz-bang toilets like the Neorest (which is in the thousands of $$ range - heated, wash, blow-dry, opens/closes by itself, etc. toilet).