If you have any problems with using fake fullscreen you'll probably notice them right away. For me it si only an issue if I want to set my refresh rate to 24Hz for movies. When I do that in fake fullscreen, playback becomes terribly choppy. Other than that I've never had an issue, and generally found that fake fullscreen ran better. I guess in theory, you might also see some screen tearing, but I've had that on true fullscreen too sometimes (though not recently).
There is no way to automatically send all OSD actions to XBMC but you can send those notifications by using a different action that specifically creates these messages in XBMC:
If you are using frodo:
<?xml version="1.0" encoding="utf-16"?>
<command id="752" name="XBMC sample notification" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>GUI.ShowNotification&&"title":"Hey There...","message":"How YOU doin?"</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
</command>