Although exteriorly the only change is the colour scheme, the GR10s have a number of differences to the GR8s internally. Improvements in the materials used are claimed to offer improved frequency response in both the high and low end of the GR10s 20Hz to 20Khz reproduction range, delivering more detail than the GR8s, which were already impressively detailed earphones. Direct comparison proves the boast accurate; the GR10s definitely do sound better than the GR8s, with subtle details in the low and high end - most obviously percussion and string instruments respectively - standing out more clearly.
From an iPod with compressed music, the difference is subtle, but with a good DAC and amplifier used to deliver lossless audio to the GR10s, on tracks that have the type of instrumentation that the improvements to these earphones highlight its definitely noticeable. It's not revelatory in the way that moving from a pair of bundled iPod earphones to a set of custom moulded JH Audio earphones might be; you probably won't find yourself noticing details you never realised where there, but you will find that they become more integrated in the mix, not a background detail you have to strain to hear.
It's probably only the type of person who really does like to tell that a cymbal hit was at the edge, not the centre, who will even consider this worth paying £400 for. Luckily for Grado, such people do exist; in fact the company's reputation for excellence is propagated by those very customers.
The GR10s have an impedance of 32-ohms, versus the 120-ohms of the GR8s. The upshot is that the GR10s are easier to drive, especially with typically low-output devices such as mobile phones and portable media players. This is obviously good because having a lower the output volume from your playback device makes the introduction of distortion by the amplifier less likely.