Set aside recumbents. How about small-wheeled uprights? The world upright speed record was set, and has held for 20 years, on a 17 inch steel bicycle with full suspension (a Moulton AM). There are strong arguments to be made that smaller-wheeled bikes allow for much faster and better designs: yet because the UCI banned them in the 1960s for being too fast, the industry was ruined and the only ones you can get nowadays are either folders (Brompton, Bike Friday Tikit) or bespoke Moultons.
BTW: recumbents can have just as short a wheelbase as an upright, as well as about as high a center of gravity position. See for example http://www.wisil.recumbents.com/wisil/racing2007/xenia2007/i... The big issue with recumbents is that humans are designed to apply torque vertically, and torque is what you need for climbing; while aerodynamics are what you need for descending and on straights.
BTW: recumbents can have just as short a wheelbase as an upright, as well as about as high a center of gravity position. See for example http://www.wisil.recumbents.com/wisil/racing2007/xenia2007/i... The big issue with recumbents is that humans are designed to apply torque vertically, and torque is what you need for climbing; while aerodynamics are what you need for descending and on straights.