the waterproof/breathability rating has nothing to do with how warm of cold the jacket will be.
here is an explanation of waterproof and breathability rating from the guys at First Drop...
what does the waterproof rating really mean?
its really too bad that there is such a large misconception about millimeters of water-proofing and breathability. the way the rating system is used is a total scam, and people really end up playing into a huge marketing scheme of companies all trying to outdo each other with tech talk.
do you know how they get those numbers? well, they take a swatch of fabric, turn it upside down (outside face down) and clamp it to a bowl. The bottom of the bowl has a tube, which runs up to a bucket that is filled with water. the bucket is then raised into the air, and the height (in millimeters) that the bucket reaches when water begins to force itself through the fabric is the mm of waterproofing.
so, even from the beginning of the test, the fabric is not only completely saturated with water, but then the increasing pressure of the raising bucket adds many times the force of gravity to the already completely saturated fabric (because a large amount of water is pressuring a small surface area of fabric). any fabric that can even withstand the beginning of this test (zero to a few hundred mm) is a great fabric.
in the real world, there is no force that puts this much water pressure on a fabric, this only happens in the lab. because of this, any fabric over a few hundred mm of waterproofing is going WAY above what is necessary for forces of water found in the real world. plus, the higher the mm of waterproofing, the less breathable the fabric usually can be.
generally, the fabrics that are above a few thousand mm waterproof tend to be woven more tightly, and therefor are generally thinner, more technical fabrics. You end up losing a lot of durability because of this. when your 20k mm jacket gets a hole in it, it's not very water proof anymore!
other things that play a big role are fabric material, construction, various coatings, seam seals and tape, lining, etc.
because of all of these factors, first drop has specifically chosen the fabric with waterproofing, breathability, and durability in mind. we feel it is the best fabric for our purposes, and we have personally tested the suits in full winter blizzards and rainstorms this year. they preformed flawlessly.
