On 10.5.8, GraphicsEnabler locks the boot sequence after the "FireWire xxx yyy is now active.." and ForceHPET restarts the machine. Both were working fine under 10.5.7. I had to go back to custom EFI strings from UInstaller.
Specs: Mobo DG965WHMKR, C2D E6400, GF 9600 GSO, 4GB (2x2) ram.