Don't do it, Scott! This is really one of the silliest ideas ever dreamed up for an ST1100. Almost as bad as drilling 1" holes in the air cleaner box. I don't know why STeinar even allows this dumb idea to stay up on his fine site. TTBOMK over the many years I've been on The ST1100 (and now ST1300 also) LiST, I've only heard of a few stupid enough to drill out their OEM exhausts.
Simply put, there is no easy, homespun way to increase power (low or high end) or sound (sadly, I assume you mean 'louder' - I'd be happy if I couldn't hear my ST at all but FWIW, I like its very unique, techno-industrial sound) on an ST1100 without spending big bucks at a professional tuner/shop, and even then Honda got it right to start with so there really isn't much of an increase even possible. If you're looking for more, maybe you picked the wrong bike.
Just MHO of course,
Regards, John