In some cases, yes. Some Linx modules (particularly the 300MHz and 400MHz products) are capable of achieving the maximum transmitter output power per FCC Part 15 and other regulatory agencies worldwide. Concentrating the power in a particular direction would, in most cases, result in illegal power levels and require the transmitter to be attenuated. This would negate the advantage of the high gain antenna to get more range. Using a high gain antenna on the receiver is a way to improve range with much less concern about regulatory issues.
For most of the 900MHz FHSS products, a higher gain antenna can be used to get more range. However, none of the modules have been certified with high gain antennas, so the final system will need to get tested and certified.
For questions on a specific product, please contact us.