Tracy, the same Oracle versions? Vastly different init.ora parameters? Up2date statistics?
Actually the CBO should be deterministic. Kinda...it has earned its crystal ball repuation because it is different from minor version to minor version.
You also don't say whether the queries on your new system perform worse?
Alas, stored outlines (asktom.oracle.com and the Kyte book have a lot on it) are not an option because the in subclauses are not using bind variables and there is simply too many different queries executed even with cursor_sharing=force.
(Have you set cursor_sharing = force ?)
