The 210 already goes as fast as a Cirrus

It certainly does. My point was that kind of speed requires HP (and a
clean airframe) and IIRC the highest power aviation diesel now is the
230hp SMA. I'm not sure if that's enough for a 180kt cruise. I'd love
to see an updated Cardinal with flush-riveted skin and the latest aero
tweaks, if that's what Cessna has in mind.