What's the performance impact of changing data type of primary key from INTEGER to CHAR(16)

There has been a debate where I work regarding the performance hit we might take by changing a primary key from an INTEGER data type to a 16-character string. Logically, the change makes sense, from an applications point of view. However, we know that we will take a performance hit... we just don't know how to estimate this. Any recommendations for estimating the impact would be greatly appreciated. We anticipate the table will grow by approximately 2.5 million records a year. If it is a problem that will not have an immediate impact--that is, it would become a problem within a few years--this would be valuable to know... we could probably expect that hardware performance will double within 3-4 years, offsetting the loss due to the change. Thanks in advance. Email greatly appreciated.