Unfortuantly there is no hard and fast rule as to how much difference it will make in your environment.
Every radio environment is different and as such the range will varry based on the environment.
The best way is to do a site survey and use the lowest power setting that gives good coverage for the areas that you want to cover but minimised covering areas you dont want to eg the car park
Some of the more advance site survey tools on the market can do predictions based on the room dimesions building matierals etc but even these tools need to be backed up with a real life site survey as they dont take everything into account
