If you want 10G performance, you need to get a 10G nic. They are only 30-40$ on ebay.
While, you CAN bond a pair of 2.5GBe ports, and POTENTIALLY get 5g of throughput, it will not be on a single session. ie- you can't download a file at 5Gbps.
10G hardware is cheap.
I was eyeballing a MD3600 yesterday, for only 150$.
Went back and forth on the idea of running it for an iSCSI san.... but, remembered why I prefer zfs and ceph over HW raid.