Your painted calipers came out nice! As someone else stated, black does tend to get lost, particularly if not a high gloss black.
The Boxster calipers won't fit the S3 discs. On standard discs they already leave a small ring unswept at the center of the disc, going with a larger disc won't give any more contact area between the pad and disc, the only benefit would be greater heat dissipation. Besides, the brackets won't space the caliper out far enough to fit over the larger disc. You can remove the dust shields behind the discs if you want to gain some cooling though.