felis:
I have a question - do you have plans to support flow control? I'm losing bytes while ftp-ing large amounts of data at high speed from Arduino.
Do you have a suggestion for how to implement hardware flow control on an atmega328p?
You may have a poorly matched baud rate selected based on the crystal speed on your arduino. Here's a nice break-down of the rate errors for different crystal speeds: http://www.wormfood.net/avrbaudcalc.php. You might get better results if you switch to a 14.7456 Mhz crystal.
The WiFly datasheet claims that you can use rates above 115200 if your packes are uniform and you ensure the last packet has been sent before sending the next. So you may have some options there.
Another problem may be that the RX serial buffer is overflowing before yor sketch empties it. Optimizing the sketch and/or increasing the size of the serial buffer may help.