I've got a lightened flywheel on my street 544. It's ok... I've got an early m40 and 3.31 rear gears, so it's a tall first...


Over all it does make the car feel a lot faster, with the disadvantages that it makes the car more prone to jerkiness in parking lots...and also when backing out of a parking stall, if you want to change direction of the car's rolling (reverse to forward or vice versa) it takes double the RPM.
Makes rev matching almost superfluous on downshifts.

As far as aluminum, I think it can be scientifically proven to be inferior. I do agree it's the best way to get below about 17-18 lbs. The R-sport catalog used to list a 14 lb steel flywheel.

*as afar as heat- you need to compare Specific Heats -ie. amount of heat required for a given change in temperature. Steel is arguably better in this regard. It is slow conducting, but can hold more heat. It also tends to last much longer without work hardening and other breakdowns - see aluminum connecting rods, rockers, and valve spring retainers- all facing overwhelming competition from steel parts. In travelling through a steel surface, then to the aluminum body of a flywheel much heat transfer ability is lost.