I have tried high gain antennae as well as directional modifications to normal and high gain antennae (basically curved foil) and have found them to be of no practical value.
With high gain, I could measure a slight increase in signal strength dB of the access point when in line-of-sight to the access point (can't remember the exact decibel figure) but this did not improve wifi range at all. Basically, areas in my flat that had spotty or no coverage did not suddenly improve or start to recieve coverage.
The problem with signal coverage in HK is not just signal attenuation due to simple distance, it is that the walls of HK apartments are made of solid concrete which completely block out wifi signal, so the signal has to bounce through doorways and around corners. Making this signal stronger does not significantly improve problems in black spots.
I have personally classed these long antennae in with other products like go-fast ram with heatsinks or racing stripes on cars- i.e. they look kinda neat but they don't really do anything.