I did read this on nasioc:

"Are there any downsides to a lightweight flywheel? While the performance characteristics of a lightweight flywheel seem to be the perfect solution, there are compromises:
a. Low end performance is affected. This usually means that higher revs are necessary for smooth starts due to the reduced rotational mass. For drag racers, this can be a BIG issue.
b. Missfire check engine lights on the WRX.

Who is a good or a bad candidate for a lightweight flywheel? Arguably, the users whose driving technique is most highlighted by the benefits of a lightweight flywheel are people who autocross frequently. Many people are also genuinely happy with the performance increase in their daily driven vehicles as well. The only group that is generally dissatisfied are people who competitively drag race as the reduced rotational mass does not lend itself to their severe launch techniques."

What the hell. I getting tired of all the bad things about the WRX versus Sti. I don't really drag race. Just a few stop light races from time to time. Mostly highway altercations!