I understand that you are using two LAN8650, isn't it? if so are they 
both running simultaneously? or you are doing the testing with one alone?

Currently we've only got one running, hopefully we've wrapped up the
board bring up in the next few days and will be able to run the second
mac/phy as well.

quoted

With this setup I'm getting a maximum throughput of about 90kB/s.
If I increase the chunk size / oa-cps to 64 I get a might higher
throughput ~900kB/s, but after 0-2s I get dump below (or similar).

Did you or possible to try a test case with below configuration?

- Single LAN8650 enabled
- UDP
- oa_cps = 64
- spi-max-frequency = 15000000,

I'll set that up and will get back to you, all testing I have performed
has been with tcp.

Did you run iperf3 test? or any other tool?
If iperf3 then can you share the configuration you used?

I just tried copying a file over the network with rsync, but I'll run some
udp tests with iperf.
I'll do my best to get back to you with results today.

I don't know whether these may audiences are needed in the CC for this 
thread. Let's see what's Andrew Lunn thinks about it?

No opinions on my end at least.

BR
Ramón