I would love to find a shorter briefcase, too. Not easy to find one that's around 13" in length. This one is 14.5": http://www.webstore.thepropertyof.com/collections/frontpage/products/wally-briefcase
The Filson 256 is fantastic. I have the navy one and love it.
I have the 257 in tan (bought when on sale from Brooks Brothers). It's certainly tough, but the capacity is unimpressive for a bag that size - my 15" MacBook Pro barely fits in with its protective sleeve.
Price is ouch.
I can't stand unlined leather. I bought this Marc Jacobs bag a couple of weeks ago from Mr Porter for much less ($400):
It's fully lined, made entirely of leather including the strap, has nice hardware including tab closures that are way more usable than leather belt-and-buckle closures, and it can take my MacBook Pro despite being significantly smaller than the Filson 257. I happen to love green, but it's also available in black.
I think it's official - Filson is now charging a designer premium over the intrinsic value of its bags, like Mismo, Porter or Want les Essentiels de la Vie (I must confess I never understood the attraction of Mismo).