Needle size does matter. Superior Threads recommends a #90 topstitching needle. Here is their recommendations page for King Tut:
http://www.superiorthreads.com/shop/...t/description/
If they're recommending loose bobbin tension with Masterpiece or So Fine, I would think it has to be even more loose if you are using King Tut in the bobbin as well as on top, as it would be a much thicker thread.