Even with a strong tail wind it still appears to be about 40mph, maybe a bit more. It appears to be regulated so that's the top speed. If this wasn't the case I'm sure 400 feet up some would truly be getting over 60+mph ground speed as registered by their GPS.
Would make sense that it would also be regulated in the standard GPS Mode. If the ground speed is more then 20mph the optical avoidance would have a difficult time braking in time.
I took a look at the pitch displayed in the .dat file while moving with a tail wind. At just over 40mph the pitch was not as steep an angle as the pitch with no wind at about the same speed. If I remember correctly, the pitch with no wind, or into the wind was 32 degrees. With the wind in the above case I believe it was 25 degrees and going about 40 mph. Not sure if this is always the case, but it would make sense.