I'd probably want to do the CPL exams for my own knowledge anyway, can never learn too much, and I would imagine it would be more beneficial for the student as well...
I don't think everyone would agree with that statement, but you'll be more employable and "might" learn some worthwhile things. You'd be better reading stick and rudder until you know it inside out and back to front, and getting some varied flying knowledge and hands-on experience with other knowledgable pilots, flying interesting types etc as you build hours.
I think you're misguided to think having passed CPL knowledge would be of any benefit to your students, or would go above and beyond what you'd learn on an FI course. A thorough understanding of the PPL theory might though.