Oh no.... Phil has mentioned Reynolds numbers.... That`s me wide awake until I know what they are....   

[50% (!)]

 

It's basically a factor of sizing vs speed and how that affects airflow.
A plane with 100's of meters of wingspan doing 400mph will have very different airflow compared to attaching even a completely perfect scaled down copy of it's wing to a car doing 50mph. To the point it will basically do nothing if you're not careful.
All my wings are based on heavily modified Selig high lift/low speed foils - which are what's used on things like low speed cargo drones and small heavy lift/short takeoff tundra/artic planes. You'd never, ever consider those for an airliner as you could never shed enough drag from them for efficient flight, and they'd cause detachment vibration at speed even if you could.
  They're also designed for a very benign loss of lift when they're beyond the ideal angle of attack - which is vital on a car with long travel, soft suspension that's always pitching back and forth.

As I said earlier, about the best/easiest thing do do would be a front splitter under the chin of the bonnet. Next best thing would be to vent the front wheelarches.

However - I have probably the worst front arches in the world for grabbing air, probably the lightest front end and the hardest accelerating buggy here - and the front end lift only really becomes an issue at 90-100mph+
So I still suspect some/most of it is because of the geometery/alignment/ride height balance.

I wouldn't bother with weighted front bars - if it it really is from aero lift it's barely going to affect it - lift will square with the airspeed so even if you put 50% (!) of the entire buggys front weight hanging on/in the tube, you're only going to move the light feeling from 50mph to 61mph...