I think that for practitioners who are tasked with actually implementing DSP techniques in a product, attainable performance of hardware targeted toward signal processing are a very relevant topic. The real problem with benchmarks, however, is one of relevance toward your application. By their nature, performance benchmark data is ruled by the marketing departments of IC manufacturers; the whole purpose of publishing the data is to increase the number of units sold. Because of this, you usually see synthetic benchmarks like Dhrystone MIPS or unattainable theoretical throughput numbers like "total number of MACs per second." These are of dubious value when it comes to actually evaluating a processor to see if it has the beef to handle your application.
In my experience, the best way to see if a particular part is a good fit for your application is to dig in yourself and evaluate it in some depth. This might consist of analysis of the architecture's instruction set, peripherals, etc. to estimate the number of cycles that your algorithms and I/O will require. Even better, most manufacturers have embraced the fact that making low-cost development hardware available is a good way to promote your platform; nothing beats getting ahold of the real article to actually verify that your requirements can be met. DSP platforms of all kinds, from microcontrollers to large microprocessors to FPGAs, are becoming more and more amenable to rapid prototyping early in the design phase.
And that is where I think the goal of making dsp.SE a place for publishing benchmarks becomes difficult. There is an almost incomprehensible number of combinations of hardware platforms, design constraints, and target algorithms; it's simply not practical to assume that you will be able to service everyone's performance estimation needs. Sure, there might be more common questions like "what embedded architecture is best if I need to evaluate as many 4096-point FFTs per second as possible?", but I would expect those sorts of pointed questions to be rare (but I would still say they are on-topic). More commonly, I would expect a software radio designer, for example, who is trying to determine whether his OFDM receiver can be implemented in real-time on a specific processor. I can't imagine a general-purpose benchmark that would give him sufficient faith to make a hardware choice; instead, good-old-fashioned analysis of the details is needed.
In summary: I have no problem with hardware-performance questions. They are a fact of life for those implementing DSP for a living. I don't think dsp.SE can practically be a storehouse of authoritative benchmark data. I also don't see this as a problem; sure, it points people away from the site, but so do many answers that reference Wikipedia or cite other sources. People who ask questions here shouldn't really care whether all of the answer is posted here; if they get a quality answer, then they will come back next time they have a problem.