There are a number of what look like reputable companies advertising simple-to-install electronic upgrades that are claimed to increase the power output and economy of turbo diesel engines. They also claim that they are easy to remove and are undetectable once taken out - although they offer their own warranty, suggesting that manufactures take a dim view. The engine of my 2010 Audi A6 is said to be increased from 168 hp to just over 200 hp. Can all this be true? How do insurance companies respond to such upgrades? A reasonable extra charge or a massive hike?