Friend and I flew a 181, 2 up, on a quick 30 mile trip. When I climbed into the back seat, and he re-trimmed to compensate, it was good for another 10kts IAS.
That's very interesting. We have two 28's at our club, both essentially the same although one is heavier than the other due to a lot more avionic stuff being on board. I always assumed that the heavier one flew slower because it was heavier (bit of a no brainer!) but the CG on the heavier one is way forward of the lighter one. The lighter one is a good 15kts faster for the same power setting, I wonder if it's because of the reason you mention. I'm intrigued now and shall get fat blokes to sit in the back next time.