I'm pretty sure that to get 1 Watt you have to use direct sequence or frequency-hopping spread spectrum under FCC part 15.247... Just sitting on a frequency as a 15.249 device or general ISM device you aren't allowed to use much power at all.
From my reading of the rules, you are limited to broadcasting +36 dBm EIRP (30 dBm [1W] into a 6 dBi antenna -> 4W EIRP). This can be a single frequency. In the 900 MHz band, your bandwidth is limited to 902-928 MHz (US limitations, different in other global regions).
You can happily beam out a 915 MHz single-tone CW carrier at +36dBm EIRP all day and that is fine.
The reason your gen2 reader is frequency hopping is not FCC mandated but protocol mandated. The UHF RFID gen2 tags have a high manufacturing variation and the RF frequency response varies greatly between tags. Some tags may be resonant closer to 902 MHz and some tags may be resonant closer to 928 MHz. Hopping over various frequencies allows the reader to address all tags in its view.
The only reason the readers are "required" to do hopping is to conform to the gen 2 protocol. In fact, from what I remember you can use the LLRP (low-level reader protocol) to stuff the frequency hopping table with a single frequency so that it will stop frequency hopping.
(My PhD was on high-data rate (up to 100 Mbps) rfid/backscatter communication.)
See FCC part 15.247: http://www.gpo.gov/fdsys/pkg/CFR-2001-title47-vol1/pdf/CFR-2...
I work with UHF Gen2 RFID readers all day which do 1 Watt/30 dBm into 6 dBi = 4 Watts EIRP, but they are definitely required to do hopping.
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EDIT: and after looking at the 50 mW 900 MHz XBee units, they are doing hopping as well according to their datasheet.
So it looks like if you want to keep the range, you may want to start looking at hopping algorithms...