1. You can't turn K800 into a "W"(like K750 to W800).
2.Yes, you can flash it to K810, but I think it isn't worth the trouble. You can change the menu, but you can put flash menus.
3. I don't know much about hardware, but you can change acoustics, camdriver and display driver in order to improve the sound quality/volume and the pictures quality .
4.See the appropriate threads and allways save your GDFS, when you start modding !