Q1) Does enable application visibility on any SSID/Interface will impact/overwhelm the switch? Consider the switch is in an enterprise environment deployment.
Q2) What is the benefit of having application visibility enabled?
application visibility wireless clients - that is part of assurance and telemetry you get if you using DNAC or Prime.
you would only get benfit for the end device connected ports, the AP connected ports you wont get much information other than AP contacting WLC here as per i know.