It's worth keeping in mind that lean manufacturing and all the interesting things that Toyota did that get written up are about building cars. In software the equivalent process is compiling and deploying code. Writing software is equivalent to designing, prototyping and testing a car. So while there are many interesting lessons to learn they are about the deployment and running of code, not about writing it. In the mass manufactured automobile industry you spend more time and effort building the product than designing it. In software you spend much more time designing (specs, coding, testing, all that stuff) the product than deploying it. The lessons of lean manufacturing may not apply to design.