Depends how you define I/O. From the applications perspective, that is the rate its reads and writes are being serviced. So as a (logical) service rate, it has some validity. And yes, a perfect example of the benefits of caching.
There are benchmarks that will give you the performance of your system, but not for your own application. These use direct I/O to (deliberately) remove the caching from the equation.
blktrace is a block device driver tool that shows the actual I/O merging and subsequent physical I/O call. Now supports per-partition monitoring, so that might be an option if you have all your data on particular partition(s) - with no other files on those partitions(s).
There are user-land tools for it, but I haven't looked at it since it was beta code, so I'm not aware of the current status.