Just did a reinstall of iPC 10.5.6. I used the Mac Pro NVidia drivers, not any of the injectors. After the install, I downloaded 9f23installer.pkg. Boot up with the basic setting. I used kext helper to install my wifi, so it created /Backup Extensions.

cd /System/Library/Extensions
sudo mv GeF* /Backup*
sudo mv NVD* /Backup*
run the 9f23installer
use osx86tools or efistuido to add the efi string for your video
reboot -f -v

Mouse lag should be gone! You MUST remove the kexts before running the installer, otherwise it won't work!