I noticed this as well. I also see it when I have my 70-200 f/4 fully zoomed-in to 200 on my 60D and there's something bright like blue sky in the frame; even at f/4.0 you can see the texture of the focusing screen. The standard Ef-A screen is quite bright, but also a bit grainy indeed.
I now switched from the Ef-A to the Ef-S screen, and it's much better. This screen gives you a bit darker viewfinder, but in return you get much better contrast and it shows the actual bokeh better. It's more easy to manually focus with this screen, which would be an advantage with the Samyang lens. If you use it with a wide aperture lens like 1.4, the viewfinder still has plenty of light.