furuno chose to go with a closed chart format for the tzt3. all of their charts are re-created by their subsidiary company, mapmedia. so the charts need to be converted to mm3d format, which is not industry standard you could say. so what ends up happening is navionics will not allow their data to be sold to a third party, in this case furuno's malmedia. so as a result mapmedia cannot make custom versions of their maps for the tzt3.
the gp1971 uses a map format that allows the genuine navionics chart card data straight from navionics, to plug into and run on the 1971. it doesn't need any translation into the garbage mm3d format.
so unfortunately without a major firmware and software complete revamp allowing a different chart format, the tzt3 is screwed on wide coverage shaded relief charting. if you have cmor in your area, you may be in luck however.
i doubt there is any chance we will get a shot at cmap's reveal shaded relief either since they are owned by simrad. we will see.