Hi. Here is what I have to say about the comparison between both programs.
Is using Talend Integration Suite really better compared to using Talend Open Studio?
There are advanced features in the open studio: group work, joblets, automatic documentation, advanced wizards... For Integration Suite: line command, monitoring console, scheduler. Support is included with Talend Integration Suite (it is a guaranteed support, compared to the community help on the forum-already of a very good level).
On what kind of project can you not use Talend Open Studio and would be best to use Talend Integration Suite?
As soon as you start working with several people on the same project or you would like to use the editor's support, that's when Talend Inegration Suite is the right solution.
Is Talend Open Studio as performant as Talend Integration Suite?
Yes, Talend Open Studio is a full version of Talend Integration Suite comparing performance between both tools. On same jobs, you will find the exact same results with either tool. Go look at the comparison matrix between both tools:
http://www.talend.com/products-data-integration/matrix.php