Great review. Ferrari has indeed made their car lighter. And Williams isn't using electric KERS, they have developed a mechanical KERS, which has the flywheel to store the energy rather than the batteries. The developing cost of this KERS is lesser and the problem with the mechanical KERS is that its heavier than the electric ones. Williams has even sold his technology to some national bus service. (Not sure which country
Talking about Brawn GP, I'm not very sure about Honda developing KERS, but the Brawn chassis was never under balanced. If I'm not mistaken the Mercedes engine is heavier than the Honda engine, so the team had planned to add the KERS developed by Honda, they won't have had a real problem, just the need to change the weight distribution a bit. Brawn GP plans to use KERS at a latter stage this year. The weight of the car has been further reduced by 7 kgs from the Spanish with their up going developments as the team developed a new revised diffuser which helps to generated more downforce and the airflow is further simplified.