I would think it's the rotational forces that prevented higher revs. 9-9.5K is pretty crazy for a 2.5L Subaru. I would love to see what this FFT setup would do on a 2.35L de-stroked 2.5, 4mm long rod motor
But that's a pretty crazy power band for a Subaru 500+ HP and TQ for 4K rpms
Can't wait to see what this does in the 1320!