I also had a bad signal on my mk808 2nd version with pins on upper antenna. This is what worked for me.
I soldered both the sticker antenna's loose. Then I cut a piece of wire 12 cm and soldered one end to the grnd and one end to the ant connection of the upper antenna (with the pins). Time to test. And YEP it worked! 3 to 4 bars signal strength, good reception.
After that I did the same with the bottom antenna, and tested. 10 dB less strength and 13 Mbps instead of 22 Mbps I got last time (acc. to speedtest.net).
So 2 was not beter than 1. I lost the second (bottom) mod and drilled 2 holes in the upper case for the wire to pass through. Took pictures of it for you guys.
So for me no original antenna's and only 1 wire mod on the upper antenna.
Hope this helps!