Understanding WiFi Networking
WiFi is bi-directional
Use antenna gain in preference to transmitter power. Antenna gain benefits both directions of the link. The access point should have as much antenna gain as is practical:
For full omni-directional coverage: use a 12 or 14 dBi antenna with no more than 3 ft. of low-loss coax. This antenna's gain comes from creating a doughnut-shaped radiation pattern rather than a sphere as in low gain omni's.
For sectorized coverage: Choose an antenna with the horizontal gain you want and the beamwidth that's practical.
On the client device side: If practical, use a gain antenna there too. Or a USB dongle for WiFi and elevate that into a more optimal RF line of sigth situation, using USB extension cords (no coaxial cables needed).
Decreasing data speed will benefit range. If you limit data speed from 10mbps to 2mbps, you have just increased the link range.
Signal Strength and Noise
You need 25 db SNR for best decode and highest speed, that said it will work with less. I don't think you will see the noise level say anything but -95dbm. Worst case -72dbm + 25 = 47dbm but should work good with less .