Hi RoVlad - this could be your lucky day. I have news about G3S GPS performance and it's good.
Like you I own a G3, bought in November, so using the original A500 chipset, unlike your G3N. Overall a good mobile for the price but a couple of weak areas. In my case the cellphone signal is weaker than I would like and weaker than my wife's G3, which is also on the original chipset but bought in January. The second weak point is the GPS. I can get a fix indoors next to a window and obviously outdoors. This is usually with the aid of AGPS, EPO etc. but in use the signal is prone to dropout which can be a nuisance if being used for car navigation. I wanted to use GPS in my car but after trials decided it was impractical. I was even considering fitting my car with a GPS booster device.
Then one day I was surfing everyones favourite auction site. I was not really looking to change my mobile when I saw a G3S on auction from Hong Kong. I was aware of its existence and its spec so I thought why not place a bid. This I did and won the phone for £101 delivered. Result! Even better when it arrived by airmail in 7 days. It could easily have been 30 days.
My view was that I could always sell the G3 and that the Jiayu may have learnt a few lessons before marketing the G3S. I was not disappointed.
I find the uprated processor gives the G3S faster performance when surfing, the cellphone signal is improved for me, but this obviously depends on where you are and who your provider is.
On wifi my maximum speed is 54Mbps which is the G standard but my router cannot handle a higher rate than this so I cannot tell if the phone could give the N standard which is 150Mbps. Maybe it can but Jiayu spec details are unclear. I think the new chipset caters for the N standard but don't know if Jiayu have implemented that feature like they probably have on the G4.
Anyway, enough of all that. I now have a phone where the GPS is acceptable. I have only had it for 2 weeks but have done one 10 mile navigation in my car that was completely successful, with no signal dropouts, albeit on a sunny day. I now am able to get a fix indoors away from a window and obviously outdoors. I just tried getting a cold fix outdoors without using AGPS and it took 80 secs initially from cold. Subsequent fixes could be got within 5 secs.
Normally, I use AGPS and EPO assistance and an outdoor cold fix can be expected within 20 or 30 secs.
I have taken advantage of certain Apps to aid this process. FasterGPS is used once to ensure your mobile uses the correct NTP server (ie. for UK in my case). Normally, I also use GPS Test for firing up the GPS, selecting EPO and AGPS, and for clearing old AGPS data before updating with fresh AGPS data.
One other bonus for the G3S`is that full HD 1080 video can be shot, although it is quite hungry on storage in this mode. Not to mention Jellybean 4.2.1 which in this case is a Lewa version.
I took a stab in the dark by buying the G3S. I am not disappointed.
I hope this info is of some use to you, even though I have rambled a bit.
Cheers for now