More of a problem is that the receive antenna on the RFID implant is rather tiny because the implant itself has to be small.
So in my opinion even a large antenna with a lot of transmit power is going to have a hard time picking up the tiny modulation signal such a tiny antenna would be able to send back.
The usual RFID cards (usually 125KHz) use the whole outer edge of the card to form a coil antenna, this gives them way more usable loop area. Yet even those typically only read about 2 to 25 cm from a reader (depending on how good the reader is). You can probably construct a special long range reader that gets perhaps 2 or 4 x this range. Even that would be fairly useless for locating a cat. At best you could possibly create an array of receivers around your fence to detect the cat leaving your property. An array of smaller antennas with separate receivers has a easier time picking up the tags modulated signal.
That being said if this is a cat that is typically outdoors, then you probably don't have much reason to worry. They remember a pretty wide area around the house and will eventually find a way back and if they are hungry they will find something to eat. It's more of a problem with house cats that don't know the area around the neigborhood and don't know how to hunt and defend, they might have a hard time finding it back on there own. Tho sometimes people adopt a random cat that wanders by and care for it (especially if the cat has a tendency to act nice to everyone like ours, he can get just about anyone to give him food, and he uses it to his advantage)