A heavy forwarder is aware of props, and therefore does compression of data before sending it. It's really hard to determine your bandwidth requirements, since it is completely dependent on your environment, what you're logging, is it raw log files, or scripted inputs, how many forwarders, what type of OS, chattiness of logs on that forwarder, etc...
That being said, Splunk is quite tolerant of network slowdowns/outages, especially when using multiple indexers and doing auto loadbalancing, which is highly recommended when you need to scale out indexers, or to ensure that your data is always being received somewhere.
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