Thanks, Dan. When I cut the jacket from the coveralls, I cut through the zipper, leaving about 5" of zipper left on the lower half of what would be cannibalized and made into the chaps. At this point, since the zipper was cut through on the jacket, you need to sew across it at the bottom so the slider doesn't come off, essentially turning the jacket into a pullover with a super long zipper.
Or, one can do what I did and unpick the zipper from the jacket starting from the bottom...going as far up as you want then cutting the excess zipper off. Then, you just overlap the Tyvek where the zipper used to be and sew it up to where you cut the zipper off. I overlapped the Tyvek 1/2" and made two lines of stitches. To finish it, run a couple lines of stitches perpendicular to the zipper at the bottom to act as a reinforcement and slider stop. Let me know if any, or all, of this doesn't make sense and I'll clarify.
I shortened the zipper like this to save weight.
As for the wind resistance, I don't know. Have not worn it in the wind yet. I have no reason to believe that it won't be good though.
Re the packed size, from just rolling it up in my hands, it'll go to the size of a grapefruit without too much effort.
If you have no experience sewing, once you cut the jacket off, you can take some of the lower piece of Tyvek and practice with it. The existing stitching at the zipper is wide and loose so is easy to unpick. And modifying the zipper may sound tricky/scary to the uninitiated but as mentioned before, this is not a difficult project. If you decide that you'd rather not tackle it, you can do what Will Rietveld did in the link I provided above and just cut the jacket below the zipper and be done with it. The jacket will be long and a bit heavier but it's as easy as using scissors.
Either route, I say go for it!