How sure are you? I'm getting the retina and the 256. The 257 definitely seems too big.
How sure? I have a 256 and a 15" retina MBP. When I put the retina MBP in either of the internal side pockets, it takes some manipulation to get in - like I said, it's a tight squeeze. If you are going to put the MBP in the center compartment it should go in fine but for me, that would make the bag a lot less useful for me. If you keep putting the MBP in the internal side pocket it'll probably loosen a little over time
If the 257 is too heavy you might consider cutting it down. By removing the interior pockets, cutting down the shoulder strap to be only as long as you need it to be ad by not using the shoulder strap pad you could save some weight. Outer pockets could also be carefully removed to get to weight of 256.