After a 3-month long process of e-mailing back and forth with Logitech support and a service center they directed me to, I finally got a replacement G203.
In all of the same testing as I described before, it does the exact same thing. It has about the same amount of acceleration as my old one.
So even though this is only a sample size of 2, I'm starting to suspect that they all have it. Since these two were bought about a year apart and the new one is also a newer revision, they either all have it, or I'm just extremely unlucky.
This one already came with the latest firmware from the factory, so sadly I couldn't test the 6k dpi firmware with it.
A pity a company like Logitech won't own up to the truth.
I've just found, using the MouseTester that the accel. is only present on certain surfaces. If I test my G203 on plain paper, I get "perfect" results. In my QCK+ Tyloo Edition, it shows the very same accel that's talked here. Can someone test it too?