There are several ways that you can do this, it's going to depend on our level of knowledge about InfoPath.
In many ways this sounds like a classic "merge form" situation. You would need to enable form merging and then when all the performance reviews are done you merge all the forms together. If you do this you may want a special "report view" built into the form that would tabulate all the scores you want to report on.
A very simple way to do this would be if you were using SharePoint, you could promote the form fields that you want to report on in SharePoint and then export that to Excel after all review forms are filled out. You would then do reporting in Excel.
Hope that helps!