have you tried the latest version of ixtreme which is 1.5, it has some fixes that improve benq reading performance.
you must be using old firmware as in 1.5 you dont have speed selections for the drive anymore.
try updating the benq's firmware as it has made a bit of a difference for me, load times seem better but noy tried gta since upgrading.
maybe your drive is just aged and needs a clean or a pot tweak, see this thread.
http://forums.xbox-scene.com/index.php?showtopic=672384