Although third-part testing is certainly best, there are a few reasons why there has yet to be much of that for MIPS (there has been some, but the results were not peer-reviewed).

First, with very few exceptions (think Zika), it takes at least 2-3 years for the entire process (initiation of the research project through publication in a peer-reviewed journal), and MIPS is still newish. Second, someone has to pay for the research, which in most cases will be the manufacturer -- a reasonable parallel might be drug companies funding research on their products. Less than an ideal circumstance, but an increasing reality.

I think there is increasing amount of reliable evidence suggesting that MIPS offers additional protection for certain types of impacts.