I wore the watch upside down on my wrist and got smooth GPS track results

I think that 1 activity is not enough in order to make a judgement here. I will try to wear to it upside down on more walks/runs to see how it will behave. However, in case it would be proven to be truth it might be handy to have a reverse mode of the display, because it is not amazing to read it, while it is upside down.

I wonder how it influences HR readings from optical sensor and step detection. 

IMHO it won’t affect the HR readings cause it’s an optical read.

no wonder about the step count

After couple of more runs I can tell that GPS track is indeed bit better. Just to manage expectations it is not really night and day difference. I would say that it is 10% improvement over "normal" watch position (in case of FR745). It is still not perfect, but it is more refined. When I run straight it will produce completely straight line instead of having some weird bends. However, in case of perfect conditions, without any trees it is same.

I would be really curious what Garmin thinks about this. I bet they must have a reason to put it where it is.

After some runs i can't see any difference between "normal" and "upside down".

i bought a stryd foodpod, it works reliably and is much more accurate

I tested this tonight and there wasn't much difference, some parts looked better and others parts worse. The total distance was only different by about 10 meters during my 7.3km run. It's GPS it's never going to be 100% correct, I wouldn't sweat it, just wear your watch the normal way.

Thanks Nick, I still haven't managed to test it more myself but this is good news as I didn't fancy wearing it upside down. True the GPS is never going to be 100% but the 935 is so so good only 1-3 seconds per KM difference each split.