yes the economy will be better if u drive at normal revs n when u drive at high revs the performance will be way lot better
exaust system will make a difference when u have headers,straight piping n a free flow exaust,no point geting an exasut with stock headers n piping,it wil be like a bottle neck,the exaust wont do any good in this case in terms of performance or economy.
my corolla 1.3L 07 gli used to do 85kms in 1000rs worth petrol now after the kit its doin 130-140 kms