I've had several overfilled bags over the years, because of down shifting within baffles from many companies. I have not noticed the same problem on stock WM bags. I talked to them directly about it , and they explained why they thought there was little to no benefit .. and I agree with them on the why. To me, it's similar to people that want to add a bunch of guy outs .. it isn't needed and for most people (95 percent or better) its a better product without the extra guy outs. In fact, we have had more paranoid users request enough that we went out of our way and then the customers reported later they wish they had not added them. Now, I agree many similar tents need more guy outs, because they use different cuts and fabric orientations .. but for Seek Outside tents ... we feel there is enough and performance in tests we have done backs that up.
In essence, I think overfill is a good idea for most bags .. but I don't think it is needed on WM , and I've never noticed shifting to be a noticeable problem .. so for me , I save the weight.