The AT instruction set v2.0.0 documentation(from http://bbs.espressif.com/viewtopic.php?f=51&t=1023) for AT+UART_DEF and AT+UART_CUR says:
Baudrate range: 110 to 115200*40 (4.608 Mega)
The datasheet for the ATmega328P shows the max baud rate @16MHz is 2Mbps so I suppose that's the theoretical maximum. I'm not sure if that will be reliable in real life. It's worth giving it a try. Please post your results. That's only if you're using hardware serial, SoftwareSerial on the Arduino supports a much lower maximum speed. I know people have reported issues using it at 115200 baud.
I've only used mine with hardware serial at 115200 just because that's the ESP8266 AT firmware default value and my application didn't require speed.