I've been looking at getting a power upgrade on my 520d and have looked at the plug-in system from Chipexpress. Power up from 181bhp to 224bhp, cost £395. I like the fact it's plugable and removable as I have a company car and shouldn\'t really do it.
http://www.chipexpress.com/products/2808/
Has anyone used this or can recommend an alternative. Comments appreciated thanks.