The standard BMW map is very conservative for two main reasons, these being warranty and emissions. To that end, the ignition timing isn't as advanced as it could be and the fuelling isn't as generous as it could be. This is down to making the catalytic converter work as good as it can - both a leaner mixture and slightly retarded timing make the exhaust gasses hotter. Manufacturers like BMW started this around 1987 with the E32 and facelift E30 with Motronic 1.3, both cars being catalyst prepared with the Lambda sensor plug in place on the engine loom regardless of whether the car actually had one. The first GB spec BMW's to have one was the 1990 24v M50 E34 followed by the 1991 E36.
Old emissions tests consisted of a car on a rolling road in a cell, started from cold and driven for a set distance and the collected exhaust gasses being measured. The cold start element meant a bit of 'Peter paying Paul' which is why they run pretty lean when hot.
Remapping a car like this E34 is just giving the engine what it really needs for maximum efficiency. Bizarrely, remapped cars are often more economical because they aren't working as hard. A 525i is better on fuel than a 520i for example.
*A properly maintained BMW engine with 100'000 miles is probably just as powerful as a new one.