Recently launched Subleger http://subledger.com/ is trying to do some or all of what you describe. It doesn't mean that there isn't room for others but consider that many early-stage companies don't have complex revenue in-flow so the core of what you're describing (converting or merging income into other operational metrics) might not have a wide appeal especially for startups.

Hopefully you get some good answers here but really whatever anyone (myself included) says here is far less valuable than asking startup CEOs what their financial painpoints are with respect to reconciling their app's internal metrics with revenue and expenses.

Finally, the question "is the market big enough" is too open-ended to answer for you. Big enough for what? To attract significant outside funding? Maybe not big enough. But to build a great income for you and a few others? Perhaps!

Happy to discuss this with you further to help you in your evaluation of the opportunity but as I say, best thing to do is canvass the potential market first.