The delivery speed does not affect (greatly) the memory usage, so you can increase it as much as you want. Put it to 10000, the plugin will stop itself if the PHP timeout is reached. Check that value on diagnostic panel, anyway: if the PHP execution time is 1 minute, for example, the plugin stop the batch after 1 minute. Since batches are ran every five minutes, you end without using all the server capabilities.
On main Newsletter configuration panel, being the server under your control, you can rise the PHP execution time to 300 seconds (the value is used only while delivering). May other things should be considered, it depends on your server configuration.