5.8 ms should be fine for whatever you're using (softsynths etc). If you have a version I card (as opposed to a version II), there is a limit as to how low your ULLI can go (I can't remember what it is offhand, but it will tell you when you try to set it too low). This affects how low your SONAR slider can go (it changes a block size setting in cset.ini, and smaller is better).
If you can get 5.8 ms without problems, I wouldn't knock myself out trying to go lower

John Braner