Ok, so somebody requested a new feature for my website that seemed pretty useful, so I went ahead and did it.
Basically, you create a best-fit curve of all processors or graphics cards as a function of price. With this, you can approximate what the performance should be as a function of price. Then, you subtract what it's predicted performance is from where it actually is. With this, you get a number telling you how far away the processor or graphics card is away from the trend line. That is, a positive value means an overachieving component, and a negative number means an underachieving component. There's another explanation on the main page.
Here's an example. As you can see, the 5870 exceeds it's expectations the most.
But take it with a grain of salt because Futuremark's benchmarks results are really inaccurate for the new cards; right now it shows the 5870 as being more powerful than the GTX 925. The CPU page is more accurate.