Is there a way to create a global property, for example, if you had a page type with a heading which was the same on every page, is there a way to make the property linked so you can change it in one place and it woul dchange it on every instace of that page type?
I know I can just leave it in the template and change it when necessary etc... but I want to be able to change the value through edit mode. without having to goto every page and copy & paste the value as there may be thousands of that page type.
If the value is the same for every page, I'll recommend to put the property on your StartPage. You can reach the property by retriving the startpage's PageData using GetPage(PageReference.StartPage).
Using dynamic properties may affect performance.
I'll recommend reading this article from one of our partner companies, http://www.epiforum.net/Codepages/Avoid-dynamic-properties-when-possible/