I expect that the vehicle speed that they attained is very close to the limit for that type of design.
The blackbird vehicle is very flimsy, they are desperately trying to keep the weight down.
The blackbird vehicle is harvesting energy from the difference in velocity between air and ground. That means that it is all about traction.
For going downwind the wheels need to have good traction; when going downwind the wheels are driving the propellor.
A wheel - being in contact with the solid ground - is capable of 100% traction; you just need to make sure the wheel will not slip.
The traction of a propellor, however, is inherently poor, because a propellor is churning air.
Imagine trying to propel an engine driven vehicle with a propellor. For a given engine output a propeller will give you far less propulsion than using the engine to drive the wheels.
In a follow-up video Derek demonstrated a device that I will call 'difference cart'
The difference cart has wheel traction both against the ground and the wooden beam above it.
By making the difference in diameter between the wheels more extreme (or by adding gearing) the difference cart can achieve a higher ratio.
For even more traction you could use toothed belts and gears.