If you are currently ranking well with your page title, you should not risk with changing it to improve your rankings. You may often see your current Google rankings go down with such a change. I myself have experienced negative results when I tried to change the title tag for a site which was already ranking well. In fact, Google slammed the site with a -950 penalty.
Your page title is the main entry gate of search engines. Changing the title meta-tag could easily make your Google rankings fluctuate. If you change your title, Google and other search engines will take some time to reflect rankings for new titles. Especially, it would definitely look suspicious in the eyes of Google and other search engines if you have made the changes in the main title and description tags after a long time.
In short, changing the main title of your site, although it’s already ranking well is not a good idea. Instead of changing the page title to try to have improvements on certain keywords, it is better to work on adding content about those keyword phrases on your page, and add links with appropriate anchor text from sites with relevant content. Google always penalized over-optimization. Now it seems as if it frowns normal search engine optimization too. So the best policy is to optimize the website at the time of creation – Do it once, and do it right !