I don't see why you would have to upgrade the number of users to get the extra features. The part number I posted is the secuiruty upgrade.See if this helps. The data sheets weren't very helpful.https://www.cisco.com/en/US/prod/collateral/vpndevc/ps6...

You should be able to order ASA5505-SEC-PL=ASA5505-SEC-PL is an option when ordering ASA5505-K8