[jakeru]

I wouldn't say it's not necessarily conclusive. If they were both linear rate springs, and you knew one had a lower ride height than the other, then you could make an inference about their relative spring rates based on this data. However, as at least the stock springs are progressive rate (and very good chance, the aftermarket "lowering" springs are also progressive, but perhaps with very different progressive characteristics), there really isn't much about the effective spring rate (under load) from your observed data.

There are also significantly different versions of springs in the rear for tiptronic cars than manual transmission cars, due to the significant weight difference back there. I would guess the tiptronic rear springs would have taller free-length than for the manual transmission cars. I don't know if aftermarket offers different springs for these different applications or not, but mismatching the rear springs for the wrong application of stock springs can significantly affect the rear ride height, and I can't imagine aftermarket springs behaving any differently. Just something to be aware of.