DSP Benchmarks for ARM Cortex-M4 processor
You're best off looking one level up, at datasheets or published benchmarks for actual devices that are available. One notable manufacturer of Cortex-M4 parts is Freescale. The Cortex-M4 is just a processor core design that is licensed by silicon manufacturers as the basis for their microprocessors. Each manufacturer designs their own peripherals and memory architecture and stitches them together with the core design. Therefore, what you might get from a core-level benchmark is the number of cycles required to execute your algorithm implemented in assembly. 

What this doesn't capture are implementation-specific issues that can affect performance, such as memory latency, interrupt response latency, etc. These quantities will vary from manufacturer to manufacturer (and in some cases, across devices in a single family). Therefore, you really need to take the entire microprocessor into account if you're looking to do something that is pushing the envelope of what the chip might be capable of.

Even better: you can often find low-cost development kits for ARM processors (examples for Freescale's Cortex-M4 family). With that in hand, you can get a very good feel for how performant your algorithm will be once implemented in-system.