I did similar without the outdoor work. After reading this, I move the antenna to our highest spot at our second floor by the windows targeting the tower. I got 13 mbps most of time also. During busy hour, I still got 7 mbps in Los Angeles metro industrial area. we are about 1.5 miles away from tower where we can't see it.
Before, we only got from 3 to 6 mbps download when we put it at first floor by the windows. Since we catch similar to the outdoor antenna concept, our noise number reduce from -81 to -60 sometimes -56 range. At this point, out ping time to LA severs is like 76 ms compared to your 50s ms. I wished someone can go up to roof and set the outdoor antenna for me what can improve the ping as well as VoIP quality.
We use most of our phony business phone lines on Skype, Skype-In and Skype-Out with US numbers. For this setups, skype is fine; not fine with Good Voice though. We are experience iffy results with Good Voice due to internet line of quality on the SIP. I just can't work this out without landline based internet.
Again, great thought and review.