I'm kind of late to this thread, but I'll post my observations and theory here now anyway.

I've also noticed the GPS speed being significantly lower coming out of turns than in straight lines, sometimes 10 or more mph slower. Also, it is not very good for quickly establishing your speed as it changes, such as when accelerating away from a stop or getting on the expressway.

I believe that this a reflection of the accuracy of your position calculated by the GPS. Every position that is calculated has what is called Circular Error of Probability (CEP). This is essentially the circle that you will see displayed on your GPS when you zoom in really close on your position. Your true position could be anywhere inside of that circle. Every time it calculates, it has that error. If it displayed your speed based on the distance per time of every position update it calculates, first, it could be difficult to read because the number would be changing so frequently. Second it could potentially have variations of several mph from one update to the next because of where you actually are in that circle, whether ahead of or behind your true position. Therefore, I suspect that the speed display is updated based on some number of points several samples apart, maybe 5 or 10, so that it minimizes the errors in the calculated speed due to the error in your actual position. In a turn, the distance between points several samples apart is smaller than the distance traveled because it is calculating based on the cord length of the arc just traveled, instead of the sum of a series of points that almost form a smooth arc. Hence, you see a lower speed on your GPS coming out of a tight hairpin turn than you see on your motorcycle speedometer.