The system I work on here was written before .net 2.0 and didn\'t have the benefit of generics. It was eventually updated to 2.0, but none of the code was refactored due to
Generics, whether Java or .NET, should be used for design and type safety, not for performance. Autoboxing is different from generics (essentially implicit object to primitive conversions), and as you mentioned, you should NOT use them in place of a primitive if there is to be a lot of arithmetic or other operations which will cause a performance hit from the repeated implicit object creation/destruction.
Overall I would suggest using going forward, and only updating existing code if it needs to be cleaned up for type safety / design purposes, not performance.