I have seen the cam timing vary a good amount as sleepyboy noted on the zetec engines. It does help a vehicle with stock camshafts if it off a good amount from factory. The article noted by sleepyboy shows a good example of how much cam gears can help by, which is a good amount.
Adjustable cam gears are a most for anyone look into aftermarket camshafts. There any many companies back in the day that used to offer cam gears for these cars. We are one of the few that still do. Link to cam gears