I think you are correctly thinking by assuming that full table scans will be faster than Index lookups because of low cardinality. The bottom line is that a certain percentage threshold of retrieved rows is indeed faster in FTS than index -I believe the threshold is (ahmmmmm???...) 20%?
1. analyze the table(s) with compute statistics and see what the CBO chooses.
2. use the /*+ RULE */ hint and force a FTS then,
use the /*+ RULE */ hint and force an Index scan
see which performs better. In other words:
Don't blame me, I'm from Red Sox Nation.