I know how you feel; I have a t5i and I'm experiencing the same thing as you, almost exactly. More than one lens looks a bit soft. Did you ever come to a resolution of your problem?
I too have a t5i and have had it too long that returning it is not an option. I also have been noticing what looks to me like poor autofocus performance with the Canon 18-135. I just brought the body and this lens in to a reasonably respected Canon...