It is very warm. And it works with either pre-washed or unwashed fabrics. If you feel very very strongly about no shrinking warping your quilt design, you can even prewash your warm and natural batt. I've done that in the past, with no problems.
I now like the slightly crinkled look you get after washing, so I don't prewash anything unless I have reason to believe it is likely to run (reds and purples).
If you are accustomed to the higher loft poly batts, warm and natural may seem very light to you, and the finished look is going to be quite different. I'd suggest sewing something small (runner, placemats, small bag) with it to get a feel for how it will look quilted.
RST