I am owner of both Logitech G25 and Fanatec GT2 and have tested them to death on my separate rigs.
Without going into any further debate I will only conclude that Fanatec still haven't been able to successfully implement all FF libraries in their firmware for proper PS3 HID-controller support.
Please do not forward me to internet-reviews and compares of those wheels because people who wrote those have all highlighted themselves incompetent with those.
Fanatec is still working hard on implementation of proper HID-controller firmware and on their Blog they even highlighted a need to develop a separate firmware for proper GT5 support.
Although I find GT2 a magnificent piece of hardware (I have CSP pedals too), there is much work needed to be done in software department. I just hope it will come sooner than later.
As Forza support via GT2 is concerned, it definitely works better than Microsoft FF wheel (I had it for 4 years, now replaced with Fanatec), but it is the same XID-controller driven FF as it was with MSFF wheel. "Better" comes only from the overall feeling, because GT2's motors are doing the best job they can with that 15-years-old software technology that Microsoft implemented into X360.
Also, this is my original post regarding this matter and it is still 100% valid:
http://neogaf.net/forum/showpost.php?p=15859760&postcount=1
Forza (both 2 and 3) force feedback effect is just "interpretation" of force feedback effect through rumble-effect. So we can't even talk about real force feedback effect, despite Turn10 did a great job in overcoming obstacles of ancient libraries on their disposal. I hope Microsoft will realize how much they would gain for their franchise and console overall by letting proper HID-support take place in their hardware.
And I can hardly wait the day when my GT2 wheel will provide me with the same amount of detail and strength in force feedback effect as G25 does.