Hi,
These are the response to your queries:-
1) Yes ,HTTP inspection will check all the connections destined to port 80 through the ASA device as per the RFC standards.
2) Might be yes , As the HTTP connections are the major amount of traffic on the ASA device , too much traffic have to be inspected by the ASA device and re-assembling will also cause the ASA device to do some extra processing.
3) No , I think you would reduce the processing for the ASA after disabling this inspection.
This would not cause any disruption in the traffic as it is not applied on the existing connections but only on the new connections which are made through the ASA device after the policy is modified.
Also , check this:-
http://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/113393-asa-troubleshoot-throughput-00.html
Thanks and Regards,
Vibhor Amrodia