To stay legal, you have to make sure the effective radiated power out of the antenna, coax and radio combunation is within the FCC rules. Cisco guarantees this by testing all their products. If you mix and match your own euipment you must take on that responsibility. Cisco calculates the coax lead loss also in the calculation, and guarantees quality all the way.
On the other hand the other antenna is probably what it says it is, but you may not get any instructions, or the antenna connectors may not be correct. Check this out first.
On another note, there is no legal way to add an amplifier to this equipment unless it is tested by Cisco as a package.
