Hi Henry,
Sorry for being slow...
1. No. The -Clustsim option is used to generate a table akin to that from 3dClustSim, and it would still be expected to use consistent uncorrected and corrected p-values across the tests.
What might vary across tests is the needed cluster size required to achieve a corresponding corrected p-value.
2. This clustering is based on permutation testing, so it isn't exactly variance that is being measured. Or maybe I am not quite sure what you mean by having more individual differences.
- rick