Partial fairings for aerodynamics alone are a tricky business as they add weight & tend to create a low pressure zone directly behind them, which can actually
increase the overall resistance through induced drag - include the added frontal area and you may well loose a bit of top speed, the larger ones are primarily for comfort & it isn't until you get into fully enclosed velomobiles that you'll see major reductions in wind resistance.
That's not a absolute ofc, much like the suggestions above - small fairings around normally turbulent areas to help guide the air flow around can indeed help, but they're fractional reductions at best, a combination of many little things contributing to an overall effect (reducing/eliminating cables, wheel covers/aerospokes, etc). Aerodynamically speaking what spandex helps with vs "normal" clothing is the elimination of flutter, one of the most severe forms of drag - but wrapped tightly or not, the human form itself isn't ideally aerodynamic, which is why there's the aforementioned aero helmets & custom "speedsuits" with trailing edge inserts, a tapered shape behind the legs & (sometimes) arms to help the airflow rejoin with less low pressure zones & turbulence.
An example would be
Markus Stöckl & his downhill world speed record of 130.7 mph;
Ofc, to massively reduce frontal area - there's a plethora of recumbent bike/trike designs, unlike your "dragbikes sans the wheeliebars" they tend to be feet-forward (not always, but usually) to help the rider's visibility (neck cramp etc) and for a more ideal pedaling position. There's some interesting recumbent plans available here BTW:
Atomic Zombie Extreme Machines! - Bikes Trikes and Choppers Plans.