I just read in the GPRS shield Ver 2.0 Wicki that the older version 1.X had 0.4 Watt RF output, but the newest Ver 2.0 now has the full 1 Watt RF output.
I believe all SIM900 modules are rated: Class 4(2W @ 850/900Mhz), Class 1(1W @ 1800/1900Mhz).
Is this a setting? Different SIM900? How can this be set/changed?