Prior to this year's Keeneland September Sale, John Stewart was largely unknown in racing circles. That changed when Stewart, the founder and managing partner of MiddleGround Capital, a private equity firm, spent $8.425 million on 13 horses. He was just getting started. Stewart returned to the fall breeding stock sales at Fasig-Tipton and Keeneland where he once again opened his check book, purchasing 11 mares and weanlings for a total of $17.35 million. The list included the two-time GI Breeders' Cup Filly & Mare Sprint winner Goodnight Olive (Ghostzapper). Stewart...