With a 256K or above connection between an RF660VPN and an RF560VPN, will I get better throughput with VPN compression turned on or off?

If the link is 512K or below, compression should provide a little more improvement depending on the data that is being sent over the VPN tunnel.

Compression is only supported in the RF6xx and the RF7xx series RouteFinders. This feature is not available in the RF550VPN or RF560VPN.