NFS limited to 4K reads/writes

I was playing around with NFS. Darwin seems to perform well as a server, but its performance leaves a lot to be desired as a client because it appears to crippled by a 4k read and write size limit. (verified w/tcpdump). I can reduce the size with mount_nfs, but I cannot increase it over 4K By poor performnace, I mean that Darwin reads at 12MB/sec over a 2Gb/sec link. Tru64 and FreeBSD can both read at 50MB/sec. (this is from a file warm in the server's cache; no disk I/O). Darwin performs just as well as the competition when its the server... Any chance of this 4K limit being lifted? Drew