Lol'd when I heard Punk say Punk/Austin is bigger than Punk/Rock. I don't see any basis behind Punk's statement besides personal bias/preference. He's a mark for Austin, has always wanted a feud with him and they'd probably have better chemistry than Rock/Punk given their history and styles though that remains to be seen since the latter haven't had as many interactions.
But besides that, Rock is easily a bigger star than Austin in this age, at all levels and I don't see Austin/Punk (if it were to happen) doing as big despite it being Austin's return match which I'm sure would create a lot of buzz. All I can say is, I doubt you could convince me Punk/Austin at WM could crack the biggest buyrate of all time no matter how flawless the build up and promotion is.