Looks like your only option would be to delete the HD version out of iTunes (you do have the option to keep the file), but that isn't really all that helpful. You could also go through MetaX and delete the catalog ID for the SD version and append the name SD to all the movies, but that would just clutter up your library with 2 of everything (though if you moved all the SD versions into Home Videos that would be less visual clutter, but you would lose the ability to sort by genres and such). It is a shame that Apple doesn't have a solution for folks who might have too much traffic on their home router to support streaming in HD and there are folks who have caps who would rather download something than watch it from the cloud (I know in your case these are not Apple purchased videos, so that's not even an option).
Have you tried restarting your router? That can improve wifi performance. As can using the 5GHz band, though you need a router and computer compatible. One other thing, if this router is more than 5 or 6 years old, upgrading it might work. I know I had issues with the wifi from my cable modem/router dropping at least once a day and having to reset the router, but when I turned that off and used an AirPort Express it has really improved wifi stability (and my PC is only 2.4GHz capable). If you have a combined unit, might be worth looking into adding a stand alone router and turning off the modem wifi.
I do wish that they had an option to stream SD, but apparently the combining of the videos into a single displayed item was meant to benefit folks trying to sync files to an external device. Guess they figure shouldn't be an option in your home streaming the HD version from a computer to an AppleTV