Holy shit. I just realized that they are Swedish too. How do they do that? I also started learning as a kid, but when I first came to Canada I made a friend at work who told me his first impression of me was that the way I spoke sometimes made the office think I'm a scary Russian gangster in my spare time. That said, even though my accent isn't really that strong anymore, it often helps, especially on the phone.
My cousin stayed with me a couple of weeks ago, he's from Norway. English was good, but accent wasn't on par with what you're suggesting. Id assume those artists work on their accent as much as they do their flow.