It does make a difference, car is certainly nippier under 3.5krpm. According to the car I have gained 1 mpg too, But i dont know. The old ones are really really heavy compared to the ams ones and the ams ones arent under driven either. But its always power to £ ratio or £ to grin factor. Exhaust and intakes usually win that but if you do have a couple of hundred quid spare then its a good upgrade.
Also belt change will be 30 mins to 1 hour but it took me alot longer to change my pulleys. Mine isnt a manual but even on other cars I have changed pulleys on its alot longer than just the belt change.