The chevette front springs in my wife's car I found by walking around a wrecking yard with a tape measure. They were already out laying on a shelf, same diameter wind, a lot heavier wire. Also a littel less overall hight in the "free" state (not installed). THe only reason I know they came out of the front of a chevette is the tag we found on the spring. The ONLY thing about these is that they don't have the tight wind one one end of the spring, but it looks like it's not needed.
The passat springs in my car I also spotted in the yard, but didn't buy because they wanted big bucks to take them out. Spotted the other car on the trailer a week later. These are one coil longer than stock mk 2 jetta springs, and are wound from a heavier wire, although not as heavy as the chevette springs. Did see some heavier ones in a "hot" passat of some description in the yard, VR6 car maybe?
Went looking because I did something similar a LONG time ago.
V6 buick and 5 speed in an Astre wagon, back springs came out of the front of a Corvair, front springs came out of a Buick Skyhawk. (Big sway bars, gas shocks, dual exhaust, etc, etc, etc......... Car liked to break spider gears until I rebushed the back suspension.
I wouldn't call this a handling "improvement", although it could be. The extra ride height could make handling worse if carried to extremes. It will allow more wieght in the back though! Trimming the spring to bring ride height back to stock would be a handling improvement.