A5c with 2 Sectors?

Will it be possible to use the A5c with 2 sector antennas to cover a wider area? If it is possible, what are the downsides?

Yes, it will be possible to use 2 sector antennas with the A5c. The only downsides are the limits of the coverage area due to the radiation pattern of the third party antenna and any increased installation costs over the A5-14 or A5-18.