GH4 with Tamron 14-150 Image Stabilization

As far as IS being "overrated" is concerned, that may be your individual opinion, but for the majority of us, good IS systems can be a life or death scenario, in particular in low light situations.

One has only to become exposed to the 5-axis IS systems in the Olympus OMD series of cameras to become hooked for life!  I have never seen an IS system that steadies the shot so quickly and effectively as that system - it's remarkable.  And allow that to translate into any lens you use makes it even more remarkable.

I too used to believe that all you needed was a tripod for stability (and a remote cord), and I used to pan IS systems because I wrongly believed that it made some photographers lazy.  Well, that has all changed.  With the quality and effectiveness of the IS systems that are on the market today (I'm not talking about the one in the GX-7, which really doesn't measure up to many other systems), they have a tendency to spoil you and they increase your effectiveness in being able to get the shot in a variety of situations (well, clear shots that is).

So to the OP, if you put the Tamron on the likes of the GX-7 and use its IBIS, you may do alright on the wide end, but you will still see "some" shake on the long end.  It's a shame that Tamron didn't make both a stabilized version of the lens and a non-stabilized version.  But then development and production costs were a huge factor with that decision.