Greylisting in v6 SMTP servers

I have observed that many v6 SMTP servers are using greylisting.  I am not quite happy with greylisting because of the extra delay time in delivering a legitimate email. If some messages are urgent, you expect the recipient can get shortly after you click the send button.  But for IPv6, can we prove that greylisting can really help since as of today there is no IPv6-based DNSRBL?