A value over 65536 is pointless and will cause you to allocate memory unnecessarily. The client maximum receive size is sent to Samba by the client and Samba honours this limit. Trying values of less than 2048 is likely to cause severe problems. In most cases the default is the best option. The performance hit will probably be greater on NFS mounted filesystems, but could be quite high even on local disks.
Some people find that opening files is very slow.
For many applications that do their own locking this doesn't matter, but for some it may. This makes things much faster. See the Makefile for how to enable this. This is because the server flushes the log file after each operation, which can be very expensive. A server may choose to not support it, however.