I've thought about RX8 seats too and a seller on craigslist had the whole interior seats for dirt cheap.
My .02 after thinking about it some more.
-I think you will sit too low for the Mz5's dash line, specially the gauge cluster. If you add spacers to raise it, kinda defeats the purpose of sporty/low to the floor seats; granted they ‘might’ huge you better –dunno.
-I'm generally not a big fan of mismatch interior seats and the shape of the Rx8’s rotory inspired seats seems kinda out of place in the Mz5’s interior.
-I second the opinion above that MS3 seats are probably a better option.