well for those of you running marlin firmware you can send an M220 S1-200. a value of 1-200 being the speed. you can just send that mid print if you want and the speed will adjust. i use s3d creator software which has a speed and extrusion jog dial on the interface to increase speed or turn up the extrusion multiplier mid print. it just sends the M220 command for speed and M221 for extrusion. im not sure if this works with other firmwares.