The chassis I have now was a McGowan barreled .308, I started out in a "accu-stock" with the full length aluminum block. I did notice a difference in accuracy, but i don't think it's because of the weight. They weighed almost the same, but the stiffness/rigidity was much noticed. It made the rifle feel solid as a rock. I think it does help that you can fit the rifle to you exactly, LOP cheek riser height etc. I also like it for the fact I can make it perfectly fit a young shooter. I have a niece that just turned 15 and a nephew that is 10, and I've adjusted it to fit them as if it was theirs and it's amazing to see the difference of how they shoot when the rifle fits them.
Since I've started this thread and the reading, I'm pretty much set on getting the magnesium chassis. With a finished weight of only 28 oz, I can't find a traditional stock with DBM/bottom metal that is close to that weight with the modularity of the chassis