So my GMG PF 4040 E has arrived. I'm rather happy with it's build quality - it looks robust, heavy duty, like professional oven in a pizzeria: no fancy design but it feels solid.
It's rather bulky with the interior slightly deeper then P134H (40x40cm) and probably solid insulation (you can touch the casing almost everywhere with the naked hand even when the oven is preheated to 500 degrees). Those additional 5 cm are quite usefull, because you can position the pizza deeper in the oven, reducing the unevenness in baking caused by the windowed door (the window is smaller than in P134).
Theoretically 450 degrees Celsius is the limit, but the actual temperature inside is closer to 500 - I didn't try, but it should be no problem to even reach 500 without any mods (it has the same limitations like P134H - when the stone is hotter than 400 degrees it's not possible to bake the pizza without burning the bottom of the cake) . From the other side it has more powerfull bottom heater (both top and the bottom are 1750W) so you can reach the maximum temperature in about 20-25 minutes.
My first experiences are very similar to what P134H user's has been reporting. Without less energy efficient teracotta stone it's a little bit hard to find good balance between the temperature of the chamber and the stone. But even with stone at 380-390 degrees and the chamber at 450-460 the result is quite impressing. It was the first time I was able to bake pizza in 90 seconds. I had no time for long fermentaion of the dough (it was ready within 6 hours) and run out of 00 flour so defenitely it's possible to achieve more than you can see on attached pictures.
Time will tell how good and problem free it really is, but at the moment I'm quite happy with my choice - the prices (with delivery) of all three: P134H, GGF Micro and GMG 4040 are very similar (at least here in Poland), but the GMG is the most powerfull of all three and makes a more "heavy-duty" impression.
Let me know if you have any questions.