This has indeed been possible for ages through ODBII, but it's costly (several hundred quid) - and while it works on a new car, once you start to get a bit older, and say, an exhaust has been replaced with a pattern part, or the air intake has been replaced with a Halfords paper part or some other modifications, then the more extreme maps (for economy and performance) may do more harm than good if they are based around OEM hardware.
There's a good reason most cars are tuned to be on the 'generalised' scale of performance or economy.
There's also the fact that if you really want decent performance out of the existing primary hardware (block, crank, cams, pistons) then you need to replace the intake and exhaust systems, and then map the engine live on a rolling road, or a test track, to ensure flatspots don't occur.
Engine mapping isn't just dumping more fuel in; to get real gains without problems (anything from worse fuel economy to blown/burned pistons - see 'mapped' Impreza WRXs for details...) then the whole engine and fuel system has to be designed specifically around being either truly modular, or truly adaptable - and if you want real, noticeable gains, you need to either start swapping hardware, or mapping live, or ideally both.
Same is true for economy - if you put less fuel into an engine - make it run lean - it will actually run hotter and damage the pistons, as the fuel, before it burns, actually cools the piston crowns.
Interesting stuff though if it can be pulled off to a useful degree.