I also saw Tiffany during the concert that Dennis DeYoung missed and I have to say she was awesome.
I had never seen her before in concert so I was not a fan but I was definitely won over by the energy and enthusiasm she brought to her show. At this age, many performers are not at their peak but I thought she sounded just fine. In fact, I enjoyed her performance so much that I would now go out of my way to hear her sing again.
Of course your mileage may vary but I thought she put on such a fun show.
~NM