I can't really see how you can get BOTH an improvement in BHP and FUEL ECONOMY at the same time.
Yes, you can remap your ECU and even change parameters - but the basic principles of energy, work and thermodynamics can't be changed. An equation is an equation.
Take the BMW 320d. There are 2 versions of that engine - the efficent dynamics, gives ~60mpg @ 163 BHP and the 'normal' model gives ~52mpg @ 180bhp. It's economy or power.
The only way to optmise an engines performance, is the ability to change parameters on the fly. The modern diesel engine, uses a mechanical cam to let in air, another to let out burnt gases and an injector to allow fuel in.
There has been a lot of interest in the developing fully electronic systems, that use electronic actuators to allow optimal air in and exhaust gases out. An engine like this offers a lot more flexibility throughout the entire operating range, meaning under varying temp, load, torque, revs etc, the exact amount of air can be introduced - unlike it's mechanical counterpart.
While a set up like this may give better performance out of the same block and more flexibility in optimising power or economy under a certain set of conditions, beyond a thershold, you won't see an improvement in power AND economy at the same time.
My 2 cents.
Eoin