1/2/3: Existing settings are not transferred. All customizations would need to be applied again in portable mode. Copying the files from appdata may transfer some settings.
3/4: You can delete the registry key and appdata folder; that is not done automatically.
5: Performance should be about the same. There are a few settings (like window positioning and import/export sets IIRC) that are always stored in the registry. Also, the user needs write access to the EXE folder (on Vista and later, programs must be run elevated [as administrator] to have write access to Program Files).
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