Mods might want to move this to the Cancun forum. All I can say from experience is at 8:45am even on a Saturday you should have no issues with immigration and customs lines, very few flights arrive terminal 3 that early in the morning. It is the afternoon when things can get a little crazy there.
I have no experience with even being offered fast track by the luxury properties in the area and it doesn't sound like anyone else does either, so if you choose the fast track I guess you will have to be the guinea pig and report back!
Thanks, appreciate the insight. Mods can move if they want, I wasn't sure if the regular Cancun board would be as knowledgable on the Fast Track services as this one.