The metric you use to track outbound banner clicks (which i am assuming are animated GIF or some other type of static banner ad) depends on how you are tracking those events.
In your Question, you said that they are "[c]orrectly campaign tracked/tagged." There are only two (commonly used) ways to track outbound links to banner ads, so it shouldn t be difficult to figure out where your clicks are being recorded/reported.
If you are tracking your banner clicks using the GA _trackPageview function (and assuming they are not Flash), then the metric under which these clicks are recorded and reported is just "Pageviews", which you can find by clicking "Content" in the navigation menu on the left-hand side of the GA Browser. The name (i.e., what appears in the first column of the spreadsheet-like reports in GA, depends on what you put in your event handler, e.g.,
<a href="http://www.some_ad_server_site.com"
onClick="javascript: pageTracker._trackPageview
( /All_Banner_Ads/Name_of_Advertiser/Banner_Name );">
Will of course report clicks for Banner_Name under the name
"/All_Banner_Ads/Name_of_Advertiser/Banner_Name"
On the other hand, if you are tracking your banner clicks using GA Event Tracking then the metric under which these clicks are recorded and reported is also under "Content", though under the sub-heading "Event Tracking". This sub-heading in turn subsumes several report options (Overview, Categories, Actions, Labels, Trending, and Hostnames).
If you used this second method to track outbound clicks on your banner ads then you should be able to find those clicks recorded/reported here (as "Events"), again, the name under which they appear will depend on how you configured the code in the page.
The data reporting discrepancy you refer to is fairly well documented. I have seen credible reports in blog posts, GA Forum posts, etc. of discrepancies from about 2x to about 10x. In sum, significant discrepancies (3x and above) are not uncommon.
The primary sources of this discrepancy are likewise well-documented.
