Are you keeping one of the desktop computers for the heavy lifting?
I traded my 17 inch top of the line Macbook Pro last summer for an 11 inch Macbook Air, as soon as the new Air was able to have 8GB of RAM. I got the 2.0gHz i7 processor upgrade an a 256GB SSD. It's plenty fast enough for me, for all those tasks -- processing raw files, stitching large multi-row panos, opening and editing very large layered tiffs in Photoshop. It's my only computer at work. Sure, it's not as fast as a totally tricked out iMac or Macbook Pro, especially with the 8GB of RAM limitation, but the key is that it fits in my little camera bag. I would not trade back.
I don't know that it will be fast enough for you, of course. But I think the key thing to consider is how important the size and weight will be.