20 million is a certainty. How fast it will get there is the question. Being bundled in the UK will be a big help and if the bundle goes WW it would obviously be huge. I think Nintendo would bundle WSR WW if SonyMo or Natal gathers momentum.
I think WSR will beat Wii Fit eventually because Wii Fit+ will eat up future Wii Fit sales. Mario Kart is going to go on forever with little chance of a sequel this gen. I think WSR is more likely to get a sequel after 20m sales than MK so that could stop it. My money is on Nintendo bundling everywhere eventually (new wiimote?) giving it the No.2 spot.
Nov 2016 - NES outsells PS1 (JP)
Don't Play Stationary 4 ever. Switch!