Well, how much extra HP do you think I got from going from 5% to 15% overdrive? I don't know if I got ANY extra Hp.....and I may have actually lost a couple by spinning it that quick (but it did keep bumping my torque up)
Even if the thing is only good for 5RWHP or so, the flywheel only costs $350. It seems like people will pay that much for 5RWHP with other mods.
I would do it because (1) it's worth more power and (2) it's free power, so your engine isn't under any extra stress.
Micah