There isn't a drop, but there is a huge increase in productivity and maintainability.
There should be no concideration on performance with the templates as long as you use smart caching techniques. If you have a page that won't change much, just cache it. If you have a dynamic one, it will have to run it, but any drop in speed is more than worth the benifits of using templates.
Personally, and I'm biased, I like my extension to phplib templates for caching and moving old code to templates:
http://phpbuilder.com/snippet/detail.php?type=snippet&id=126
