Can startup experience ruin a resume?

Let's say I currently have a M7-ish resume (3.4 GPA in finance from a semi-target while being a D2 athlete; 750 GMAT; ECs with leadership positions; first in the family with a university degree; caucasion male if that matters).

My current work experience is maybe M7-ish? (3 years in corporate banking including an early promotion at a bank like Deutsche Bank). 

Would working for a funded startup (~100 employees) in a strategy position deteriorate my resume? The startup is relatively unknown; however, my work experience so far is entirely corporate banking. 

I have asked around and some say that it is a step-down while others say it is simply adding unique work experience that will boost my resume.

If my goal was to get into an M7 and get hired by one of MBB (+ Deloitte), would this be a good or bad decision?