It's a reasonable question, if not one that anyone has actually asked me. But I'm going to answer it anyway. The way I see it, there are two trains of reasoning behind my choice of test dishes; I'll call them "rational" and "emotional", because that's what they are.
My rational reasoning is fairly straightforward. The way I see it, tempura is something that is hard to do well, and fairly easy to sell even if you do it badly. Therefore, only a place that really cares about the food it is serving will bother to put in the effort it takes to make really good tempura, and that sort of dedication to quality should spill over into the other things they make.
The emotional reason is, as emotional things tend to be, rooted in childhood memory. When I was about five, my family went to Japan. I don't remember it very well, but I have it on good authority that the two things I ate the most while we were there were tonkatsu and, yes, tempura. I do recall being grossed out by the sushi, but as I remember it it was the idea of eating seaweed that got to me, not the raw fish. At any rate, it set me up with a lifelong love of the Japanese fried stuff, which I think has fed into my disappointment with some of the tempura I have had recently and driven my desire to find some that recalls what I originally liked about it so much.