Tricky question... I wouldn't give up a GX8 just to get a stop or two more image stabilization. From DPReview: "Panasonic says that the Dual IS system is 3.5x and 1.5x more effective at wide-angle and telephoto, respectively, compared to the GX7 that preceded it. It's worth noting that much of the improvement at short focal lengths is due to the fact the GX8's in-body stabilization can counter translational movement and the GX7 could not. ...In our tests we found that the GX8 gives you about 3 stops of stabilization at telephoto focal lengths (200mm equiv. in this case). In real-world terms that means you still get the majority of shots in-focus at ~1/60 sec with IS compared to ~1/320 sec without it. That's not quite as good as the 4 stops we measured on the Olympus PEN-F."

From what I've read, the GX8 combined stabilization is almost on par with the original Olympus OM-D EM-1 (NOT the Mark II) IBIS. The G85 and GH5 should be on par with the Oly OM-D EM-1 Mark II IBIS.

The only reason I'd upgrade from a GX8 would be to get a better shutter. With the recent firmware upgrade, the GX8 switches to electronic shutter mode to avoid its inherent shutter shock between about 1/60 and 1/320 second. Electronic shutters are not for every photographic situation, as they tend to have "rolling shutter effect" when the camera is panned. The G85 solved that with a different shutter. However, unless you photograph lots of action, it probably doesn't matter.

Stabilization beyond 5 stops is probably marginal, anyway. There aren't many photographic situations where you truly need more than that much help, because any motion in the scene gets blurred at a certain point (which depends on many factors). My GH4 isn't stabilized, but my lenses are, and so far, I haven't encountered situations where I truly needed Dual IS. I get 2.5 to 3.5 stops of stabilization, depending on focal length and how much coffee I've consumed.

Like all cameras, it isn't perfect, but the GX8 is a fine machine. I'd probably skip a generation before updating.

I have a GH4, and while the GH5 is a HUGE upgrade from that, I may wait for the GH6. Hmm... that's probably three years away... GAS will probably get me before then, especially if the price comes down to $1600 or so. Of course, if Trump puts a lot of tariffs on foreign goods, that won't happen... Ahh, risk!

When I upgrade, it won't be for any one feature, it will be for an overwhelming lot of them.

The GX7 has the earliest implementation of Panasonic's in-body image stabilization. Leave both stabilizers on when not using a tripod. Turn both OFF when using a tripod. You may wish to experiment with one, then the other, then both stabilization systems, with each of your lenses, to get a sense of how much stabilization you can get (how slow can you go?). Your results will vary with your physical condition, caffeine intake, mood, etc. (!).

Generally, in-body image stabilization is best with lenses shorter than 100mm. In-lens stabilization is best for use in longer lenses. This is why some manufacturers don't put stabilization in their shorter lenses.

Both Olympus and Panasonic have excellent in-body stabilizers in their very latest cameras. Most older Panasonic lenses that have OIS will be updated with new firmware that supports the Dual IS II in the new G85 and GH5 cameras. That system is good for up to 5.5 stops of shake reduction!