The "Max Throughput" checkbox is only available on computers that support Direct Memory Access (DMA). Currently, only the Macintosh Performa 6100 and 6400 series computers support this capability, and only the 6400 is bundled with a modem for which this is an option.
The feature requires that the functionality be installed in hardware; there is not a software-based patch to implement DMA. Without DMA support, Macintosh computers are limited to a 57.6 Kbaud throughput rate.