Having had a 2.5i and then a 3.0si the best thing you can do for fuel efficiency in the 2.5i is fit a 6 speed gearbox
Under 3k is my rule of thumb for petrol cars, although I never noticed that much of a difference on an economy run in the 2.5i rather than pressing on. Maybe 5-7mpg difference overall. It would manage 40mpg+ on a long run though.