I have a 6600 and bought the bobbin case with the new fmq foot. It makes a huge difference to free motion and I am very pleased with it, I prefer quilting with the finer threads. It is very smooth and quieter.
The only thing I've found though is free motion with 40/3 threads or metallic is better with the original bobbin & fmq foot (on my machine) - with the new one I was getting breaking threads using the thicker threads.