Hi. I don't think there ever was any talk about using the F4 engine as a code base for Fighter Ops. I think they did a smart choice when they made a special agreement with Austin... but then again, the only thing left from X-plane is the terrain, wich is highly tweaked already in certain areas (Southern Calif, Las Vegas, San Fransisco, Middle-East etc.). The rest is builded from scratch (afaik). I encourage you all to support them. Every flight sim needs all the support they can get now a days :)
I am impressed by the models they show us on the new web, at the same time I can't wait to get my hands on Flaming Cliffs :D.