Answered my own question. I was able to obtain closer to 10GBit speeds using jumbo frames:
[2.4.3-RELEASE][root@pfSense.localdomain]/root: ifconfig ix1.7
ix1.7: flags=8843 <up,broadcast,running,simplex,multicast>metric 0 mtu 9000
options=600703 <rxcsum,txcsum,tso4,tso6,lro,rxcsum_ipv6,txcsum_ipv6>ether 00:08:a2:0d:65:f6
inet 10.1.7.8 netmask 0xffffff00 broadcast 10.1.7.255
inet6 fe80::208:a2ff:fe0d:65f6%ix1.7 prefixlen 64 scopeid 0x14
nd6 options=21 <performnud,auto_linklocal>media: Ethernet autoselect (10Gbase-Twinax <full-duplex,rxpause,txpause>)
status: active
vlan: 7 vlanpcp: 0 parent interface: ix1
groups: vlan</full-duplex,rxpause,txpause></performnud,auto_linklocal></rxcsum,txcsum,tso4,tso6,lro,rxcsum_ipv6,txcsum_ipv6></up,broadcast,running,simplex,multicast>
[2.4.3-RELEASE][root@pfSense.localdomain]/root: iperf -s -B 10.1.7.8
------------------------------------------------------------
Server listening on TCP port 5001
Binding to local address 10.1.7.8
TCP window size: 4.00 MByte (default)
------------------------------------------------------------
[ 4] local 10.1.7.8 port 5001 connected with 10.1.7.6 port 56306
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 9.47 GBytes 8.13 Gbits/sec