For something as critical as a winch line I would go with a company I trust. Remember that synthetic rope was used in the marine, rigging, and logging industry before it was used for 4x4s. You can buy synthetic line by the foot in many diameters from rigging and marine suppliers, often at a better price than than from 4x4 suppliers, although you will need to splice it yourself. Sampsonrope.com is a good resource to learn about synthetic rope, splicing techniques, load limits, etc. They are the manufacturer of Amsteel Blue, which is a Dyneema fiber rope similar to what you referenced in your post, but they have a proven track record in many industries
I made my own winch extension line by ordering Sampson's Amsteel Blue rope from Redden Marine Supply, using the Samson Rope's techniques to splice eyes to each end, and using hydraulic hose chafe guards to protect each end. You can save some money doing this and learn a lot about your equipment in the process.
Incidentally, you can get the Amsteel Blue in 1.5" diameter with a 228,000 pound average strength if you are really hard core!
I just gotta say that you better make good and sure what you are buying has the specs they are touting. Personally I wouldn't buy anything I didn't see come off a well known factory spool with the line ID and specs on it. My truck, my a&&, and the safety of those around me are worth way more than a few buck saved.
Lots of overseas look-alike stuff out there! I saw some anchor chain from china with genuine ABS documentation stretch and fail at about 10% load (on a test bed)... lucky no one was depending on it!!!