You probably need to restart syslog-ng for a buffer size to take effect, and you also have to make sure to change the correct knobs.
There are two things to take into consideration.
1) throughput
2) latency
If the throughput is higher than what syslog-ng handles with the specific config, buffer sizes will not help.
If syslog-ng is generally able to cope with the load, one has to make sure that the input buffer size of a socket is large enough to keep incoming traffic between syslog-ng starting the poll on the socket vs it actually starts reading messages.
Also, syslog-ng may be forced to stop reading it's input if the destination blocks/slow.
Back to how long it takes for tuning to take effect: after a restart, immediately if the load is the same.
Bazsi