The SATA speed is very good, 300MB/sec.
Gigabit ethernet is only 125MB/sec
Can 1 SATA interface be converted to 2 Ethernet 1Gbit interfaces and use those interfaces in an Etherchannel ? Is this technically possible? Or maybe such device already exist? If so, with 6 SATA ports on a good motherboard you could have 1.8GBytes/sec (14.4Gbits) transfer for a cluster node for example.
As far as I know, it doesn't exist and it would serve no purpose since there are no 1.8 Gb switches. You can use 10 Gb Ethernet cards (and switches already exist for those).