I haven't been able to find any specularity numbers for this online, so attempted to test it myself with a laser pointer, screen, and digital camera. Best I could figure is about 80% specular, tho I'm not entirely sure how much I trust that.
The printing plates are very highly anisotropic, in that they reflect tightly in on direction and widely in the other. So if you bounce a laser off it you'll get a long narrow line as a reflection. It actually looks quite cool. It is nice and tight on the tight axis, about a centimeter at 3 meters distance, which is better than I would've thought. On the other axis it's almost 180 degrees blown out.
Long story short they should be fine for parabolic troughs as long as you make sure to bend across the grain of the metal, so that it's the tight axis which forms the focal line. I learned this the hard way.
We made a Solarflower in Spain last December and though didn't get a chance to properly test it all, after ten minutes in low winter sun the top (so the coolest part) of the collector pipe got to 140 C. I'm going to take this to imply the plates are at least usefully reflective.
I've also been using them to make a vertical axis wind turbine, which I'll post to the Other Tech page here and on the main site once the tutorial is done, which should be soon. The plates are really nice to work with, you can easily form precise shapes by scoring with a craftknife and giving the metal a couple of flexes, the line can then be torn out. Also, bending the edge of the metal into a triangle makes it surprisingly strong and rigid. The turbine, which consists only plates, a bike wheel, nuts bolts and rivets, was successfully tested on the back of my father's car up to 105 km/h, and it should take more.
So with that in mind I've an idea now of how to make the SF collector entirely out of printing plates, so no wood or plywood required. It should be about a quarter the weight, easier and cheaper to make, and more precise. I'll get it prototyped and update the tutorial when I get the chance, but this might be a while as I'm madly busy preparing for the trip through Asia in a couple months.
If you're planning to make anything get in contact with me first, I'll take you through the new design.