No, not made the discs smaller , but you'll notice from the picture you posted that the pad doesn't reach the inner part of the disc. So I machined it out to within about 2mm of the pad. Only about 3mm.
Chris, it is to do with the wheels. The 993tt calipers ( or big reds as they are known ) fit beneath std 17" avus, but need spacers with 18" rstts.
As triple said though, the GT3 calipers are a lot bigger. Of course, they are fittable if you mount the disc far enough in, which you can't do with the R32 discs.
Chris.