I've got it running on an iBook using 9.2.1 and it keeps warning me that it isn't tested for the newer operating system.
I've heard conflicting reports about whether or not 9 has any 68k coded anywhere - and I also read that after Speed Doubler first came out - Apple switched to the dynamic compiling for 68k code that made Speed Doubler such a boon.
I've heard too that even things like disk drivers contain legacy 68k code.
Is there any test or way to tell if Speed Doubler helps with OS 9?