I've had the Rear Differential for about 6 months now. Major improvement in divability in my own opinion. I'm in city traffic most of the time and can travel in 2nd and 3rd gear with out the down shifts into 1st. On the track, the difference is amazing! It just seems easier to find the right gear and the low end torgue is very noticable. Corners where you would normally drop down into 2nd or even 1st , you can power through in 3rd. I have not considered the Flywheel. I'm still not convinced of any benefits.