Definitely worth doing when a PID Controller has been installed.... 8-) Its not recommended for standard (t/stat controlled) Silvias though, as the insulation will create a number of headaches with operation, due to the wide deadband of the t/stats usually fitted.
With an insulated Boiler, it will become a much more stable machine and improve the overall longevity of electrical and electronic components, including the wiring. Heat is the enemy of all of this so any reduction will be beneficial. You will lose the side-effect of a cup warming tray though; heating cups with Boiler water or via some other method will have to be employed.
You also need to make sure that you insulate the top of the Boiler too, not just around the cylinder. Most of the heat loss is from the top but Ive noticed that a lot of people dont seem to worry about doing it because of the complexity created by the pipework, wiring and other devices. It doesnt take all that much effort to make a template and then cut the insulation out to suit...
All the best,