I think I know the answer to the this question, but out of interest... 

Do the fibre ONT boxes handle 10Gbit? I assume they don't?

I currently have a pfsense box with this card, "Intel® 82571EB Gigabit Ethernet Controller," and I'm wondering if a better card would reduce latency to the Australian Dota2 server in Sydney?

I'm in Wellington, on Fibre, MyRepublic, Gaming, the 950/450 Mbps plan. My current ping is 33ms and with speedtest the Sydney Internode provides a similar result. 

Card details: https://ark.intel.com/products/20720/Intel-82571EB-Gigabit-Ethernet-Controller

I considered getting this card 2nd hand for ~$100 recently. It does a few more things on chip.

https://www.pbtech.co.nz/product/NETITX540T1/Intel-X540-T1-Ethernet-Converged-Network-Adapter-1

If I get a single port card though it might not make a difference if it has to be passed to the quad port card anyway?

The desktop that is dedicated to pfsense is complete overkill, it is unlikely to cause any slowdown.

AMD A6-6400K APU with Radeon(tm) HD Graphics
Current: 1800 MHz, Max: 3900 MHz
2 CPUs: 1 package(s) x 2 core(s) (1 to 10% utilisation)
AES-NI CPU Crypto: Yes (inactive)

State table size: 0% (269/3190000) Show states
MBUF Usage: 0% (5570/1987926)
Load average: 0.31, 0.26, 0.25
CPU usage: 8%
Memory usage: 1% of 31901 MiB

I have enabled net.inet.ip.fastforwarding=1, but it didn't make any difference. All my cables are Cat 6a.