Sgt_Kickaxe - 5:56 am on Mar 5, 2012 (gmt 0)
Two years ago I noticed my site had crept up to page 4 for a keyword that webmaster tools suggested my site ranked highly for despite not being part of the core subject. e.g. blue widget grease instead of blue widgets.
I added the word to the title in what I thought to be a clever way and promptly lost that page 4 ranking for the word(down 500 places) and lost a good chunk of the rankings I had for previous words(down 50).
I though "wow, that was bizarre, but no problem I'll change it back". That should have worked right? wrong, the site never regained the page 4 rank on the non-important keyword and the other pages have recovered a bit but are down about a page each.
Why? What happened? In my opinion the change prompted a re-evaluation and, perhaps, a human editor to come along and decide the site was definitely not about the extra keyword so a manual action ensured it would not rank for it.
THAT action has the potential effect of negating a lot of incoming anchor text which in turn also sinks your site a bit, perhaps even more than the human review intended.
Bottom line - can you improve your rankings with a title change? Only if the keyword was glaringly missing from previous titles and SHOULD have been there. Otherwise you'll see no improvement at best and a lot of loss at worst.
Remember: Google monitors top results for most keywords with human evaluators so if you creep up into this area you'd better be able to pass a sniff test. Keep that in mind if you suddenly want to rank for a new term otherwise it's a fast pass to the end of the line for you.