performance tuning

Hello

In our environment (oracle v 11.2.0.3 on exadata platform)  I am seeing a number of queries that perform better utilizing a full table scan as opposite to using indices. Whenever I see a SQL with performance issues, the "fix" seems to be to make the indices invisible and run a report after setting optimizer_use_invisible_indexes to false on the session level.  This all sounds fine and great, but it also looks like a brute force method. I am concerned that having more and more queries using FTS will eventually exaust even exadata's resources and lead to a big system-wide performance issues.  The