There are such device but for cars and saw it in a British automobile program 'Fifth gear'. But they only work because the engines are MPFI. People in India made MPFI engine in bike a failure due to its high cost and complication, only KARIZMA ZMR has MPFI, but u cant connect the device to it.
Mileage and power are 2 things which are totally opposite to each other. If a bike gives u power then it will take more fuel, so less mileage and vice-versa.
But today automobile companies add more valve to the engine so both mileage and power are balanced. For example, Discover 125 ST and Vespa LX 125. In 125 cc scooty segment, there are several scooties, but Vespa is the only one which gives the highest power and gives enough mileage like others, this is because of its 3 valve engine. So this way even a 100 cc bike can give more power than a 115 or 125 cc, it is possible.
U can balance this too, but only by tuning the engine. Its simple. By tuning u can make a 100 cc Splendor to race or a Pulsar 150 to give high mileage.