Yeah, at a certain point you can only extract so much from a mash, so you're going to be running up against a wall. Let's say you want to make a 1.05 10-gal batch. To get that, you'll have to wind up with a 1.1 5-gal batch, which is pretty hard to get, and you tend to be lucky if you get about 65% efficiency with a beer that big. Bobby's plan is probably the best. If you don't want to buy a new kettle, though, you could always treat your batch like a 10-gal partial mash. Mash a batch at a point where you've had a decent efficiency in the past (say 1.07 final gravity), then do a late extract addition to make the rest of it up. At that point you could dilute it just like people doing a partial boil/partial mash batch.