Couple things, as I am currently doing the Seasonic X-1250. The wire is 16AWG. You can use this, but it's hard to work with, especially while trying to crimp connectors. I recommend buying 18 gauge wire and making/crimping your own wires. This way you can also control the length.
Seasonic has an odd way of wiring things, so like lutro is saying, you'll be spending some time on atypical wiring solutions.
Also, the crimps that seasonic used are a biotch to remove from the connectors. You will need to wrap the wire around your hand to get enough force to pull them out. Every single one has been like this for me. Once you crimp your own with MDPC crimps and crimper, you will see how much easier and cleaner they are to remove, and also how much better of a crimp it provides. The seasonic wiring job leaves much to be desired, but you'll find this attribute on most PSU suppliers. It works, but not the prettiest.
One other thing, the sleeving, heatwrap, and glue is a biotch to remove on the stock wires. Imo, you're probably just better off buying your own connectors and just tossing the seasonic ones. Or just save them for a rainy day.
In fact, for the ATX cable and others, this is what I'm going to do. That way I can always have the stock wires in case someone wants to buy my PSU down the road.
Good luck with whatever you choose. Sleeving is rewarding, but it's not as smooth as these guys make it look. While not hard, it takes a bit to get it down.
This was my first cable. It took me 3 hours to get done. lol.