Faster 25 fps is a much better looking stream at that bitrate/settings
faster 25
veryfast 30
Think of the bufsize as a little video clip where you're telling the encoder to target the maximum file size.
If you specified a maxrate of 1000 and a 3000 bufsize you're telling the encoder that the maximum file length is about 3 seconds and you want the finale size to not exceed 3000 kbits, that's the basic idea. and the benefits is that it should be a little more efficient in terms of bits/quality ratio compared to three 1 second clips at 1000 maxrate each depending on how complex the same scene is.
If you're worried about extreme spikes you could try this test
viewtopic.php?f=18&t=2613 and switch between a static scene and the test scene to see how the spike should affect your connection.