C41W could be both wireless and wired. Then the customer/tech just hooks it up in the most convenient manor. A wifi chip should not add much to the cost of an RVU client (although who knows how much more directv would charge for it).
Maybe it will. But don't forget that they also need to make room for internal antennas in the box which might also change the physical size of it. Plus when you're talking about hundreds of thousands, if not millions, of boxes that "doesn't add much to the cost" starts to add up.