I mean Dr Du is good full stop, you'll be getting people who got 99s and 100s in their HSC 3U/4U math exams as your tutor regardless of your class, but I would recommend you strive to move up classes by being top of B, top of A3, A2 and getting into A1 eventually, there are several people who improve rapidly at Du. Or even just getting into A3 is a good aim.
My reasoning for this is that you won't get dragged down by a class, rather brought up to a higher standard by a higher class.
Wouldn't recommend Dr Du if you're not prepared to put in time and effort however.