Measure the distance from the bolt hole the the spring perch. That is what matters. I believe it will be the same. A longer piston won't matter, because the spring will dictate the distance between the perch and the upper point where it mounts to the car. The only time the shorter piston will matter is if the suspension is at full droop.
I put two red lines in this image. They are exactly the same length. It seems that the perches are the same so I measured off the top since they are spun 180 degrees. I think that the 2010 is shorter due to the new design on the bottom of the strut.
I'm starting to wonder if the rear lower camber arms are different from 2008 to 09-10 to make up for the difference. Or if it's all in the springs.
