@SomewhatSane
Hello!
Try to enlarge buffers to 1 MB or even 2 MB. Directives to set OpenVPN buffer size:
rcvbuf x
sndbuf y
where x and y are in bytes.
For example, for 1 MB buffers:
rcvbuf 1232896
sndbuf 1232896
It must also be said that, in order to beat 500 Mbit/s, you need some luck, i.e. you need finding a server that in some moment has a very low bandwidth requirement by other clients.
Also, if you have an AES-NI supporting system but a less powerful CPU, try AES-128-GCM.
Kind regards