Foreign Key For Index Column - MYSQL
I might be wrong, but I don't think that the MySQL optimizer takes foreign keys into consideration. An index, on the other hand, may be very useful as a support for a foreign key. 

However, the main purpose of a foreign key is completely different than what you are asking for. The purpose is to keep the database consistent and maintain referential integrity. When deciding for or against foreign keys, I think this what you should be concidering.