KIX immigration can be very quick, depending on the time of day, as it's often not very busy. I'd make a plan and if you get out of immigration within a certain time go for it. If delayed, then hang around the airport.

Kobe ferry is only 30 minutes, add 10 for the shuttle bus to the ferry.
http://www.kobe-access.co.jp/en/timetable.html

With decent conditions, you could potentially arrive in Kobe within 1 hour 15 minutes of landing and get a boat ride as a bonus.