Actually, it is a terrible idea to add an identity to this table.

Depending on your specific data, future inserts could be sequential anyway. Even if not, rows are inserted once and typically read 10s, or even 100s, 1000s or more times, particularly for lookup tables, which this certainly seems to be. Besides, tables can of course be rebuilt when truly needed, and rebuilt online for this table (assuming Enterprise or other qualifying edition).

Overall, the biggest performance gain you'll get is to stop using dopey identity clusters by default and instead consider carefully how the table is accessed. I've removed over 6000 indexes and improved SELECT performance dramatically, particularly for joins, the majority of it from just replacing identity clusters with properly-keyed clustered indexes.