Anyone have an idea if it is possible to install Windows 8 on the F90? I may purchase an F90 and would like to upgrade the OS if possible.

While anything is possible given the right resources (time, money, information) there's a huge hurdle to overcome: drivers for all the devices on the F90 (boot support code, Bluetooth, commands to/from the support chips on the radio board, etc).  Without those, you can't really do anything from the OS.  Pioneer developed all that code in-house for the version of Windows running on the HU, and they're not likely to share with homebrewers.

If you want the latest and greatest OS, the F90's not your best candidate.  Best to purchase a newer model, or make yourself a carputer.