Originally Posted by r3dn3ck

Agreed in principle but, that would heavily depend on the diameter and sectional density of the starting and ending parts. In some cases an aluminum ds will have identical or even more rotating mass (above a specific rpm) than a steel DS.

Case in point:
In one instance, I had a 4" aluminum DS and a 3" steelie. The steel was 8lbs heavier but due to the location along the diameter of the bulk of the steel DS's weight; which was completely contained within 3" of the center, the aluminum had higher rotational energy at it's curcumference after some few thousand RPM, since almost all of the weight of the aluminum DS was more than 3.5" from the center line. The centrifugal forces were higher despite the literal weight being significantly less. This is all 100% academic but it shows that design and diameter are as significant a factor as gross weight.

While teals math may not be totally accurate he brings up an important point that works both ways, so be sure of what you're getting and what problem you're solving with your mods.