I thought the Win audio model had changed so that this was not required any more.
Not a resource hog in Win 7, nor in 8 even with the included AV component. Still handy to have for protection against VST(I)s et al that one has downloaded. Not likely to be activated in the middle of recording or listening, unless you decide to interact with something external right at that time. If you do turn it off, you need to AV check EVERYTHING that you import and export on some other machine to cover your butt!
I do NOT recommend disabling the internal soundcard, as then the OS WILL use your nice fancy audio interface if it really needs to make a noise, which is obviously NOT what you want happening in the middle of a session.
Retaining the onboard soundcard active, with system sounds off, allows the OS to default to it if it needs. Some programs may ignore the system sounds setting during installation and create audio events with active sounds, but if you don’t have the onboard audio going anywhere, you will NEVER hear them. Typically, they are programs, such as communications ones like Lync, which you would not normally have in a DAW, though you might if you wanted LAN based comms between the studio and the control room.
Of course, if the drivers for those onboard devices are creating problems, then disable them, but check what sound events newly installed programs may have decided to provide.
The ‘High performance’ plan is already included with Windows, and you cannot delete it, so you only need to edit it to ensure everything stays on with no power saving options.