I have a courtney 5.6kg jobby on the scimitar build, I've no idea how it feels yet as it doesn't have a prop shaft and many other things, but I can tell you the revs pick up very quickly indeed.
There is a massive advantage in haveing a light flywheel, you can run through some simple calc (simple if you make some resonable assumptions).
Assume the majority of the mass of the flywheel to be at 2/3 of the radius to make life easy, so imagine the flywheel to be spinning a large mass at 95mm from its centre.
We'll work with 1kg to start with.
Multiply this 1kg on the flywheel through the first gear ratio of 3.81, and the final drive ratio of 3.9 (if thats the one you've got) and its like having an extra 14.6 kg strapped to you're wheel at 95mm from the centre for every 1kg on the flywheel.
You can now equate this to an extra mass in the car by multiplying this mass by the ratio of "mass radius on the flywheel / radius of the wheel".
So in first gear its like 4.4kg extra weight in the car for every extra kg of mass in the flywheel.
And if you shave 10kg off with a courtney flywheel its like loosing 43kg off the weight of the car in first gear.
This lowers to a 24kg reduction in car weight in 2nd gear, and 15kg in thrid.
I've not done this calculation before, and I seem to have almost convinced myself while writing this that is might not have been worth the extra money

I was a little conservative with the assumptions, the majority of the mass is very near the outer edge making it more like 55kg of the weight of the car in 1st.
hmm..... I kinda hope I've got something wrong, I was under the impression it made a bigger difference
Bummer!