I have had the SPEC and McLeod on my own 2005 Mustangs. too light and the harmonics drove me insane.
EXEDY Racing Clutches has just released their new billet steel flywheel with a complete redesign and reduced moment of inertia and the damn thing rocks! We picked up 11rwhp from it and it has zero harmonics and raps up fast and still has a decent return. All this and its only 3lbs heavier than the aluminum versions. Its all the design and the MOI!!!
Shoot me an e-mail and I can get you images of the unit installed and even get you the best pricing!
eric@impulsecars.com