I've been testing FI with P3Dv4 for a few weeks, made hundreds of hours worth of flights and it works really well. Whilst the frame rates suffer a little bit due to the complex nature of the beast, it is still more than flyable.
Whats more, all the instruments are completely legible, and just by leaning forward a little, you can program the FMC without even having to use the zoom function!
Make sure you enable mouse acceleration in the Maddog menu, to help turn the autopilot dials.