I went through a short stage where I tried to pay attention to writing style and writing quality. Slowed down my reading quite a bit, which didn't bother me. It also lessened my my enjoyment level which did bother me. I tend to get lost or immersed in a book. The more I am enjoying the book, the faster I read it.
Generally I read pretty fast, which actually used to annoy me slightly because I don't enjoy rereading. Now that I have an ereader and instant access to almost any book I might want to read, it is overall a good thing.
I don't think most speed readers, whether natural or those who have taken courses, always skim or read as fast as they can. I read about 300WPM on an ereader, slightly faster with paper, and my average score on reading tests online, is 480 WPM. Entrance exams, or aptitude tests are quite a bit higher, 600-700 WPM with 80% retention, but I really tried hard on those

I can't see enjoying a book or even retaining an interest in what I am reading at those speeds.
I think that taking a speed reading course will up your reading speed without lessening your enjoyment, as long as you don't consciously try to read everything as fast as possible. And unless studying a lot of material who cares how fast you read. It is interesting to know, but that is about it.
Helen