GX7 with legacy teles
The in-body stabilization in cameras like the GX7 (and better viewfinders) make it more practical to make use of the tele lenses that we once used on our SLR's and DSLR's. I found the Canon 200mm f2.8 L lens to be particularly useful. For long tele applications I usually get better results by using that lens with digital zoom rather than by using longer lenses (300mm Tair, 400mm Sigma APO, 600mm Perkin-Elmer).
One exception was an old 600mm Tele-Tokina. The screen image below compares the 200mm Canon L (with 2x digital zoom) with the 600mm Tokina (with a 2x optical converter). The images are scaled to provide the same magnification.