Some questions about old_snapshot_threshold

Hi,

1. The following presentation uses serializable transaction isolation level to force a snapshot too old error.
(http://momjian.us/main/writings/pgsql/features.pdf)
Is it possible to get a snapshot too old error with active old_snapshot_threshold when the only used transaction
isolationlevel is read committed? 

2. Does activating old_snapshot_threshold have a negative impact on peformance?

Regards,
Marc-Olaf