Most databases are mostly concerned with random reads, normally 70% of their activity. This leads to lots of disk seeks, and the key performance metric is IO/s. You say you only care about the throughput of big writes, though. For this, iSCSI is less ideal than some non-IP option, however can be made to work.
iSCSI is limited by being non-duplex, as reads and writes share bandwidth. If you want to maximize throughput, you will want to have as large a dedicated IP pipe as you can get to your iSCSI disks. Potentially you could use more than one cat 6 cable, or even a 10Gb/s link. Your maximum throughput would be between 80-95% of line speed, based on the amount of packet collision overhead you get.
A fibre channel connection would go 8Gb/s per path duplex. That means no matter how much returning traffic (scsi acknowledgements, etc) you have, you will always have 8Gb/s of write bandwidth.