1. The IE80 has a lot more mid-bass than the IE800 does, which ultimately clouds the mid-range. Sub-bass wise it's the other way around, and because the IE800's bass focus is on the sub-bass, the mid-range remains unaffected. So does clarity.
2. The IE80 beats the IE800 in term of forward projection, but the IE800 offers depth that almost makes the IE80 sound 2-D. Imaging "angle" (sorry could not find any more proper word) is wider on the IE800, making for a more precise instrumental placement.
3. I personally found the IE800 to be much better than the IE80 in most sonic regards. But also IMO the IE80's midrange presentation strikes quite a shocking resemblance to the IE800's, minus the added depth and resolution. I also think that the IE80's sound signature, with its warmth and excellent forward projection, is actually capable of bringing a better sense of immersion.
There are also the usability issues with the IE800 with its microphonic cable and the cable-down wearing style, so I do advise you to try them out first if you can to see if it's worth paying the premium and enduring all the hassle for sonic upgrades over the IE80.