Am I right in thinking that changing a title tag causes a temporary drop in Google SERP or did I dream that? If I change the title tag as part of targeting a new keyphrase then should I see a temporary dip in the results for that keyphrase? And for how long? thanks!
It depends on how big you have made your title tag and it stays on topic. Even if you have cluttered lots of keywords in title tag you may not get proper benefit of title tag. Ideally I would say 2/3 keywords is ok.
Yes, changes in title tag can causes changes in SERP of your site. Title is one of the elements that affect SERP.