It all depends really. New instalments of dev kits usually bring with them a more tweaked, improved element than the last one - say, a less power-consuming, slightly more powerful CPU.
It's worth noting, however that these changes aren't massive alterations to the performance of the machine - in many ways, yes, they keep the same foundation, and build/tweak it to the final specs.
However, the difference between a first generation developer kit and a second generation one is likely to be far greater than, say, 2 to 3.
Put simply, the difference between the first build and the final, production specifications end up being quite a bit different - it just comes in smaller steps.