Patrick, Your quilt turned out extremely well. I like how you thought through the modifications to make it unique for you. Eliminating the top cord should work just fine for a 40 degree quilt and the custom length also reduced some weight. This allowed you to go up to M90 (a great material) with very little added weight. Though I am partial to the carrot look, I must say the brown is earthingly pleasing.
On the temp rating, you should be able to measure a solid 1.5" of loft. This is generally accepted as enough to get you to 40 degrees YMMV. A bivy will also help at the edge of the range. This quilt really shines in the 50's - 70's because it is so versatile...drawn close, open, under you, flat, etc. It also makes a nice blanket to wrap around you while sitting in camp. Though I have not done this in the field it does layer nicely under a 3 season quilt to extend a 3 season quilt's range.