What will the difference be?
I believe that leaving all the shields you use at “normal sensitivity” is the way to go (except Internet Mail and Outlook/Exchange), it provides well balance between security and performance, getting high sensitivity doesn’t make Avast! recognizes more malware than it normally is.
Furthermore, getting normal or high sensitivity has no effect on some providers (e.g. Network Shield, Web Shield, IM, P2P), it’s just the interface goes, the sensitivity level works on Standard Shield, Internet Mail and Outlook/Exchange only.
For Standard Shield, Normal is the default and provides a good balance between performance and protection, I would suggest having the Internet Mail on High.
Is it really so that High setting only affects Standard Shield, Internet Mail and Outlook/Exchange, like Maeng says? If so, the user interface should indicate as much (in my opinion).