Bunta drops off a little if you manage to pull through the first hairpin at >130. Try to go through most of the corners >125.
Bunta is tackle corners at good speeds so cornering is the emphasis. You can't really do much with the straightaways, usually you pull away a little after a well tackled corner onto a straightaway but you'll notice as he gets up to speed, he will slowly gain on you again. So concentrate on making clean corners and using the gutters appropriately.
At least this is what I noticed

By the way I play the InitialD Special Stage/Arcade Ver2/Arcade Ver3 but I am better with IDSS than in the arcade. I still have got lots to learn from the guys.