[mst]

One dude tells me, FF gains 2 stop compared to µ 4/3. In the scientific equation of that matter sensor age didn't play much of a role...

Wait a bit, and let that dude quote himself:
 

My point was: you're ignoring ISO, because on the full frame camera (assuming similar resolution and sensor on roughly the same engineering level
), you just bump up ISO by two stops, because that will lead to the same image quality in terms of noise and result in the same shutter speed, too.

Of course the two cameras you mentioned (D3x, D7200) do NOT qualify here. I happen to know very well from theory and practice

In this context though, (again) recommended reading:

http://www.photozone.de/Reviews/986-equivalence

Klaus explains things in there very well.

There is no "one truth" in this discussion. There are a few basic facts which lead to the theory of equivalency (in terms of DOF, FOV and ISO). This is honestly just physics (and not the rocket science kind). There are of course many parameters that you can not calculate in, because in the end no two camera sensors are really 100% comparable, just as two lenses aren't, even if their specs are very close or even identical on paper (otherwise this site obviously wouldn't exist).

It's a personal matter of taste how much you take from the equivalency equations or if you even care about it at all. It does, however, give you an estimate (and a fairly good one), what to expect in general if you intend to compare systems. For some the drawbacks of a smaller sensor might be ok in daily life or even welcome (talking about DOF for example), for others, they are a major concern and a reason to not consider m43 (count me in here).