You need to use a bit of common sense whe processing. If you only have 20mins and 300 emails or a 3inch stack of paper in your inbox then you need to make some compromises. To get through it all you you can only allow your self 30secs of doing time, off loading the rest onto next actions lists.
The 'what is it?' Question exsists to give you a pin point focus, it seems obvious but simply asking that question, out loud even, makes your brain comes up with answers with what to do with it immeadeatly in the background.
What's the next action will naturally follow next, etc
I find to stop overloading with the 'what's the outcome?' question is to apply the classic GTD - two minute rule and if it not complete put the next action down on your lists. You'll find your brain will automatically give you the outcome and for a lot of smaller things and you won't need to add these outcomes to a project list, which avoids overwhelm. As long as you complete the next action on your list, and if the outcome is still not achieved, cross that off and add the next action to that, etc and keep going you will motor through your day.
The key is to always replace a next action, when you've completed it, with another one when the outcome is not achieved. This is instead of looking back to the project list at weekly review time to add the next action. It cuts out a lot of rethinking time as once you've completed a task, right at that moment, more times then not you know what to do next with out thinking to much about it.
I am finding that while doing this through out the day I can keep 7 or 8 things going which are not on my ever increasing project list. It takes seconds to add a next action just after I've completed the previous one, but I can jump over to something else if I need to and keep a lot of things moving at the same time.
I've recently found doing this breaks the processing overwhelm i face every day. I am one of these people who finds an opportunity in everything and my project list grows and grows. As long as I keep my next actions list fresh I can power through things at a phenomenal rate instead of adding things to the project list with a sense of dread as it gets longer and longer.