[Paul]

Your line will only support a maximum speed which is based on your distance from exchange, quality of line, quality of equipment on line and contention to name but a few things. Your ADSL service will have adjusted when you first connected to the router by finding the best possible speed without errors or stability issues, there is no point your line running at 16Mb if it is actually corrupting all the data that is being sent/ received.
To know what you are likely to get we would need to know your attenuation level, distance from exchange etc. The link to adsl kitz will give you a good estimate of your speed, if you are too far from exchange moving to ADSL max will make no difference as your line may not support it.