I liked the Heidenau K73 also. The front was fine, the rear indeed had a slight strange shape. Flat in the middle and very steep at the sides. Never really got to the edge of the rear tire (chickenstrips). Never had any handling problems, but they did spit me off the bike once, at very slow speed with a damp road, a little bit too much lean angle and a bit too much throttle (and perhaps the white paint on the road also helped).
I never got to try them an a Supermoto though.
They did get me up here though:
I guess there are a few examples here that 21/18 is'nt neccesary to have a great off-road machine. I guess 18"front and back works also. Of evne 19" front and back. Or 17"s
I wish i could get Kenda or Shinko in my country through my normal internet tire place. Really would liek to try some of them.