Having run the ABF flywheel around 5.5-5.6kg for a few years, I can't see a downside & the response is everything.
Interesting is when say a Mk2 is fitted with an ABF, but retains the 020 box, the flywheel can go down to something in the 3kg range, and still it is hard to see a downside!
Definately contributes towards waking the powertrain up