Do a follow up post some time and let us see what you discovered.
One thing you will probably run into if you try to do long distance RFID reading is the RF in the testing environment effecting the ability to demod the RFID. This is due to the ability of the receiver to filter out the out of band signals. Whatever out of band signal that gets through the filter effects the AGC of the receiver, which can bury your RFID signal in the mud.
You can see this effect easily with a remote control car fob. If you are in a quiet RF environment, the remote has greater distance than if you are in a noisy (urban) environment. I've had trouble being near comm sites and not able to get the remote to work.
Getting long distance reception for weak signals in a noisy environment is an art. You can add filters before the AGC, but loss in the filter directly effects the noise figure. For really critical signals, you generally find a low loss cavity resonator right on the front end, then some gain (but with overload margin), then a lumped or saw filter before hitting the AGC.
When these manufacturers state a working distance, they make a lot of assumptions. The more hardware you throw at the receiver, the more distance you can get. In 2010, the defcon record was 217ft. This does get tricky since ISM is by definition a shared band.