Looking at last season's stats first-class stats, these appear to be the best allround performances (minimum of 4 matches).
George Linde (6 matches):
345 runs @ 49, HS 148*
30 wickets @ 21, BBI 7/64
Jason Smith (8 matches):
452 runs @ 41, HS 85
11 wickets @ 35, BBI 2/44
Senuran Muthusamy (5 matches):
242 runs @ 30, HS 80
21 wickets @ 27, BBI 7/36
Jon-Jon Smuts (6 matches):
319 runs @ 35, HS 75*
22 wickets @ 26, BBI 6/50
Other potential names which come to mind are Dayyan Galiem, Marco Jansen, and Gerald Coetzee (although those last two may be more bowlers who are handy with the bat than true allrounders).