A tried and true technote covers the buffers associated with NBU: http://support.veritas.com/docs/244652
If you could provide some additional details such as whether you are backing up to tape or a disk storage unit that would help with some recommendations. If you are using tape the size of the data buffer should be set to the value recommended by the tape drive manufacturer and will be 128, 256 or 512kb. The number of data buffers is related to the amount of cache memory that your particular tape drive has.
Right now I have a Solaris test box running NB6.0 to an internal fibre disk storage unit and it sustains 48-50 MB/sec. NB6.0 will send data to a backup destination of tape or disk as fast as that tape or disk will write it. If you provide some more config details I will try to add more recommendations.