I don't think it's going to come anywhere close to the FTW shows.  However, it seems like Bob has gotten his sh*t together over the last year.  Hopefully, that will hold up.  I don't know much about Mayer other than (as you said) he's supposedly a really good blues guitarist.   I predict he holds his own, but I'm not really expecting anything magical to come out of this.  I think anyone expecting Chicago Part 2 is going to be disappointed.   I just expect it to be a fun time with some good music.  The excitement is back in Deadheadland.  That goes a long way in making a good time out of this.
Chicago was over-the-top fantastic.  From here on out, everything else is just a bonus.