hey mate. ive got an ef fairmont, standard diff ratio, thinking of going up to the 3:45 gears. ive heard there is hardly any difference at about 100kph, (maybe 250-300rpm), so its not much.
depends how high ur going in ur diff ratio.
and yes, you will definitely notice the difference in take off power off the line, quite a bit seeing that the XR8 already has a higher ratio ne way.