If you're not interested in your work enough to spend 8 hours a day on it, you're doing the wrong work. (Or the sucky part of the right work... we've all been there.) I don't think it's possible to do your best work on something non-trivial without fully immersing yourself in it.
This is simply not true, the 8 hour workday is a complete arbitrary concept. There is a wealth of research to show our creative thinking is best done in short spurts. Time off in between actually helps your subconscious continue to work on the solution, something known as "diffuse mode" when I am attempting to solve a problem. There is a reason why so many of former geniuses, great authors, mathematicians said their best solutions came after a walk through nature.
Maybe I should have phrased it as "to spend 8 hours a day thinking about it." And I'm not suggesting you need to be spending every waking moment on it, either. But you have to be focused on something for some minimum percentage of your time (which does vary for each person) before your subconscious really starts digging into it.
I'd agree with your final statement, but I don't see any connection between "full immersion" and eight+ hour days. Rather the contrary, actually. Three or Four hour stints of concentration are optimal for me, and seldom seen to happen in an office environment.
Clearly, others will do their best in different environments. The search for "one size fits all" is part of the problem here.
