depends on when in its lifetime it released? launch window/mid life/end of cycle?
if it released in the first few years I think it could have possibly pushed GC over 25 million and edge out a 2nd place finish over Xbox. if it released in 2005/2006 I think it would have been too little, too late.
regardless, it would have one of, if not the best selling title on GC. somewhere around 7-10 million.
a $99 NSMBU bundle for holiday 2003 could have been a pretty big hit.
When the herd loses its way, the shepard must kill the bull that leads them astray.