Oh gawd, not again! There was nothing much wrong with the original method. All it needed was a bit more emphasis on 'you will need a personal estimate'. All DSL ISPs forced the user to go through an estimator before you could sign up. There is no point trying to accurately predict connection speed for DSL at the national level. Your connection speed is a characteristic of your line and your line only. And even knowing the connection speed - so what? It's not like it will be any different if you change ISP. Most problems are close to home and will affect all DSL based ISPs equally.
What might be useful would be some kind of measure of consistency. Something that can indicate contention levels at peak times. That can be a genuine differentiator.
Anyway I also think it's sad if people are basing buying decisions solely off adverts. I agree that they shouldn't mislead but anyone that takes what an advertisement says at face value needs taking to one side for a quiet chat. Never trust anything any advert says. Even better - never even read the damn' things.