Saw a "Rolling road shootout" type thing in a Vauxhall comic the other day. A couple of people with stock engines getting a fair bit more than the engine's meant to produce (say 15 to 20 horses on a 130bhp engine), and lots of cars with backbox, filter, perhaps "stage 1" or "stage 2" tuning from various people, generally getting a fair bit less than their "predicted horsepower". All the people with stock motors were saying "Vauxhall clearly know their stuff, making engines this good".
Nobody seemed to think that the figures were more than a little optimistic...