I love the Big Wreck cover as it does intersting things with the original. They made it their own.
Dream Theater covering "Xanadu" felt like a weird Rush tribute band but with a karaoke singer doing a histrionically bad Geddy Lee impression. Odd combination that doesn't work IMHO. If your singer can't do a full on copy of the original song, then make the whole song your own- which is more interesting anyway (see Big Wreck). Put a twist on it in some way with arrangement, keyboard sound choices, guitar tone choices, etc.