I'm not a high rep user on SF (yet!) but I am on SU. And the answer, quite simply is, you don't need to answer all that many questions to be a high rep user. On SU, I went for one good answer a day, maybe 2. On superuser I have 1571 answers for 39,111 rep in a total of 1063 days on the site (for comparison, I have 1934 rep on serverfault for 79 answers over 718 days). At some point of time I hope to be able to answer more questions here, but its still a factor of skillset over time.
I have a few tricks - I love RSS feeds for sites I'm new to - its a good way to get a quick overview of active questions including the content of them. Many also recommend specialising in specific sorts of question.
You don't need to spend lots of time to get rep. You just need to work smart.
As for employment prospects - I think this answer is a pretty good alternate view. Its a matter of how you sell it - reputation means little, but community involvement and varied quality answers may show an enlightened employer you have skills the same way a reference would.
Finally, a smart sysadmin keeps his employer happy, and works as little as possible. I will leave the implications of that as an exercise to the reader ;).
