The power is limited by the spectrum regulations, and I think 8W is way too much.
This kind of magnetic antenna is used by radioamateurs not having enough space for a full size antenna. l remember in radioamateur handbooks it said it's not safe to stay near a magnetic antenna during tranmission - at least a few meters.
The antenna appnotes for magnetic antennas say distance=coil radius and this is true for usual size centimeters.
However this is true because the antenna and card/transponder size are quite compatible and the coupling is good. The card is much smaller than 1m, so in this case the coupling may be inefficient, only a small part of the energy will intersect the card. Also the effect of the card modulation would be hard to observe on the powerfull antenna signal - the receiver coupling is problematic because of high power.