Why the CIK went with 5″ specifically, and not 6″, I don’t know, but I can imagine why having tested them back-to-back. The 5″ wheel, with the taller tire side wall, talks more to the driver and gives feedback before loosing lateral grip. The 6″ will let go without much warning. There are some classes in which the 6″ works well, such as laydowns as dirt ovals, but not with high HP classes, or those that have a very narrow and/or peaky power band.
I’m not sure I agree. The old Firestone YFx tires were pretty darn good and the sidewalls were only 0.25″ shorter as the tires were 1/2″ bigger OD at 11.5″. I’d offer that tire availability is what drives the 5″ size decision by today’s racers and that ‘business realities’ are what killed off the 6″ tires in the first place.
You might want to venture down in into the 250 Superkart section of this forum….they use 6″ wheels. I’m sure they could give you a different take on them and the availability of them. The CRG part may be an issue though because of their front bolt pattern, but I do know they make 6″ wheels in standard and metric sizing.