The usual settings of both OpenSolaris as well as Solaris seem to be optimal for sending data across that link as long as you remain in the 1Gbit/s region, but are not optimal for receiving. The quick tests showed with default settings ca 900 Mbit/s upload and 250Mbit/s download speeds.
To check your current settings have a look at the following variables (the last one seems not to be defined in OpenSolaris):
ndd /dev/tcp tcp_recv_hiwat ndd /dev/tcp tcp_xmit_hiwat ndd /dev/tcp tcp_max_buf ndd /dev/tcp tcp_cwnd_max ndd /dev/ip ip_soft_rings_cnt
Increasing the TCP buffer sizes (the system has to have plenty of memory though) can be achieved with:
ndd -set /dev/tcp tcp_recv_hiwat 524288 ndd -set /dev/tcp tcp_xmit_hiwat 524288 ndd -set /dev/tcp tcp_max_buf 16777216 ndd -set /dev/tcp tcp_cwnd_max 16777216 ndd -set /dev/ip ip_soft_rings_cnt 16
This sets the default buffer sizes to 512KB and the maximum buffer size to 16MB. It should be easily handled by a decent system and should give a decent performance even at 300ms distances and multi Gbit/s speeds. In case memory issues appear you can reduce the usage by halving the numbers.