20's will get you around just fine in most cases, and are much cheaper
A 400ex will get you around just fine in most cases as well, and again - much cheaper.
But you bought a Raptor, not a 400ex. Why ? It costs more, but it's better. You put a Sparks pipe on it, why not an Yosh ? It costs more, but it's much better. NO REASON to go with 20's at that point IMO. They cost less, but they perform less well. Why get the best of everything then take away all that potential with a pair of 20" tires ?
That's like buying a Turbo Busa and putting a governor on it is the way I see it.