I agree that there isn’t any proven performance superiority. Icarex has a long proven track record, though, and some people will be willing to pay for a known quantity. Others are cost sensitive and may well be getting the better deal. Given the small size of the sport kite market, it could be a couple more years before there is enough track record to say one way or the other.
The only kite in my collection that might be PC21/PC20 (I’ve seen it called both) is the Sky Sport ATM. Honestly I don’t know, but it’s never worried me. It performs well and there aren’t any unexpected wear issues.
The funny thing is, Icarex was short lived in the sailing market it was made for. Icarex spinnakers were known to fail unexpectedly and catastrophically. Boat owners didn’t care for that. It makes a nice kite, though. 😉 I’d be willing to bet there are other spinnaker fabrics being made that are as good or better for kites than Icarex, but that name sells.