I gave the TGD71c a try on piano last weekend. The group consisted of piano, 5 string electric bass, and two vocalists. The piano was on high stick.
The single mic worked well, picking up a nice balance of the range of the instrument. However, we had a problem with low energy regeneration on stage. The bass was getting into the piano mic. When I muted the piano mic the problem went away.
There is a bandshell on the stage which may have contributed to the problem.
I like the sound of the mic on piano and it's ability to do a good job using a single mic but it is probably best used for a solo piano concert, piano and vocalist, or an acoustic ensemble.