Just did a really quick search and I found something:
"The engine upgrade, as Schnitzer agents JSN Motors calls it, consists of a Schnitzer management chip. It costs some R12 300, but it does have the advantage of retaining a full BMW warranty, something that very few engine tuners can boast about."
See
http://www.cartorque.co.za/0140.htm.
It's not a diesel and it may be pricey, though.