In my experience, the range was a whole lot better, even without adding additional antennas, if you upped the voltage on the transmitter. However, it didn't do any good using the voltage through one of the Arduino pins (e.g., Vin). Running it just off the Arduino at 5V (via USB), I would get a range of about 6ft/2m. If I plugged a 9volt battery into the Arduino and still fed the transmitter via Vin, there was no difference in range. If I powered the transmitter directly from the 9v battery, the range increased to more than 30 feet/10m.
Bob