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OK, here's an exposure push test series to check for shadow desaturation.
Taken under (icky) compact fluorescent lighting, with quick "custom" gray card WB. SPP "neutral" color settings, NR defaults (middle setting). All at ISO 100, varying shutter speed in 1-stop increments, then pushing results in post (first 2 stops in SPP, rest in Aperture) to keep lower-left white square at fairly constant luminance.
No attempt was made, beyond the gray card WB, to get colors "correct" in this, which might have been futile under these lighting conditions because color accuracy was pretty awful to start with.
-0 stops (nominal "correct" exposure by camera, a bit lower than I would have manually chosen):
-1 stops, pushed +1.24
-2 stops, pushed +2.27
-3 stops, pushed +3.27
-4 stops, pushed +4.6
-5 stops, pushed +6.4
-6 stops, pushed +8.0
Color saturation never completely goes away, but it drops pretty steeply at -4+ stops under. There is definitely also a strong nonlinear "S-curve" tonal adjustment being applied (as Tariq has noticed), based on the increasingly large amounts I needed to push the files (compared to the shutter speed, especially at -4 and beyond) to keep constant exposure; the bottom row of gray patches also shows a steeper fall to black at lower exposures.