I doubt there are a lot of tactical downvoters hitting the rep cap often. It seems to me that the people who spend a lot of time on here are more respectful of the environment that Stack Exchange continuously tries to encourage. The people who spend a large majority of their time asking and answering questions on Stack Exchange seem more interested in creating great content than downvoting something purely to make him/herself look better.
The people I've encountered who would resort to desperate measures such as what you describe generally have very low rep and struggle to get upvotes because they write very crappy answers. When I write answers, I tend to edit them as much as I can to make them stand out against competing answers; thus, if I ever do get tactically downvoted, it doesn't matter because I know I'll get 5 to 7 upvotes from all the people who actually care about Stack Exchange looking good to people who are legitimately looking for expert answers.
Also, other people who see a great answer get downvoted are more likely to respond in kind and upvote your answer. People love controversy, so a tactical downvote by a lone desperado is likely to work in your favor, as long as your answer really isn't deserving of the downvote, that is!
No, there's no need to shorten/lengthen the lock period; I just don't see this as a huge problem. My suggestion is to focus on great content. Make your answers look better than everybody else's, and one lame downvote won't make a difference in the long-run.