This may not fully answer your question, but as a general rule, running any database in compatibility mode is simply saying that, although you have a higher version of SQL Server installed on the server, the database will only have the features and options available to the compatibility level you've chosen for it. To my knowledge (and experience) there's never a performance penalty or gain for running in compatibility mode. You just lose features.
This is particularly helpful when consolidating databases/servers and/or you need to run a 2000 or 2005 only supported database on a SQL Server 2008 platform, for example. Now, upgrading your databases to 2008 will obviously get you the latest/greatest features, so you'd always prefer to run in native mode (the level of the SQL Server binaries installed) to get the best bang for your buck.
So to answer your original question ;-), setting your compatibility level to 80 would just be like running your database on a SQL Server 2000 platform (albeit with better tools :cool
Patrick Purviance, MCDBA