Is academic success in an undergraduate degree predictive of career success in the field of psychology?

I don't think it is. Success comes from you. You may be academically successful, but when you enter the real world, it's a totally different thing. You can only be successful if you follow your passion. What is your passion by the way? Remember that success will follow you if you follow your passion, that's a fact. Because if you don't, no matter how successful you can be as how other people see, if it isn't your passion, then you will never be happy, that's again another fact.