I would prefer to make this a comment, but I don't have the rep yet.
I work in the U.S.A. in software startups; if you are working in a different industry/sector or country, my answer may not be applicable.
I also like to find out about the financial health of a company when I am interviewing. I usually preface my question with, "I know that you are private and that financial information is sensitive, but..." and then ask about burn rate, how long they can last at their current ops level, etc. Most companies are willing to share, in rough numbers, this information. I agree with David K that revenue per employee is not something most interviewers will have at their finger tips.
As far as it being a good idea to ask, I think it's entirely reasonable for an interviewee to get some idea about the state of the company with whom he or she may be working. If a company is very guarded about this info, it's a red flag to me.