<p>Except perhaps at bigger (or better) shops, the development UI is done by the developers.  In your experience, how much has this impacted the final product, and how much time should we spend getting the development UI right?</p>

The interface you're building and showing to your customers is basically all they will ever see. You can sit down with them and talk for hours about what the product is supposed to do, but the number one best method to have the users spot problems or missing functionality, is to show them the interface as it will look like in the final product. You can do this with a prototype or the actual interface as it is being built.
So I would say the interface should look pretty much like what you will be delivering at the end during development. This will guarantee that you get the right feedback, and that the user knows what he is getting. Of course, you can build an interface which doesn't exactly have the style of the final product (colors, window styles, fonts, etc...), but the functionality of the product you're building should be clear as early on as possible.