Restriction on the rad itself, not in the case. Your rad is getting less air through it compared to if it didn't have the filter on it. It might not be incredibly restrictive and you might net 1C from taking it off, but I'm all about being cooler and having more performance. If it wasn't such a bugger to disassemble it and test, I'd suggest seeing if there is a difference.
Oh, and just looking at the pic, you might want to try putting the filter on top of the chassis, it would be easier to clean. Just a suggestion for when you eventually have to take things down to clean.