For me, the results speak for themselves. And i think this design can be improved. I would take away the arms,and 4 out of 6 pulleys,thus reducing a bit of it's size,weight, some friction, cost,and stuff to break. I would take away the plug with holes for the cylinder,which lets atmosperic pressure push the plug. I think that cap with holes,slows down the airflow into the cylinder.
The rail could be lower placed on the cylinder,taking away a bit of weight,and size.
Here's a shooting video at 50m. If someone has an audio/video editing program,it would be possible to calculate the average velocity of the arrow at 50m by looking at the time from the launching sound to impact sound. But to me it sounds about 1 sec to get to 50m and those are probably the 55 gram arrows,described on their page. That's impressive power..