Really cool story John, thanks for sharing!
The wider tires are a necessity due to 2WD regulations unfortunately, I recall both the EFRA and BRCA state that a 2WD car should use wide tires on it's driven wheels. However, for the FF it may indeed be beneficial to stick with 4WD front tires for various reasons.
First of all because of the whole front suspension, because I can then important a 4WD front suspension onto the car. This makes it cheaper, easier to develop (as there are more driveshaft and front hub alternatives out there). The car would have a smaller scrub radius too (distance between tire contact patch and steering pivot of the front wheels), and the dampers could be placed further to the outside on the suspension arm. And when you look at the tires themselves, narrower tires can't balloon as much as wide ones - especially of soft/clay tires - which means the actual contact patch under hard throttle doesn't actually differ much from wide tires and is more consistent than wide tires. For high bite tracks a narrower tire may also work because it corners easier - after all your diff helps to turn your car more easily, but that only evens out the necessary difference in rotation speed of the left and right wheel, it does not correct for the different rotation speeds that would be ideal throughout the width of each individual tire.
This combination of lack of ballooning and narrower contact patch is what helped the first prototype's pace a lot on clay during the test in Bergschenhoek last year. Note the pictures in
this video, the front boots are 4WD front tires stretched on 35mm wheels (so slightly wider than 4WD front wheels, but narrower than modern rear wheels).
As for the K1's better performance over an FWD Durango, I think it's down to a set of factors. For one, the belt drive should help to 'give' a little, acting as a damping feature in the drive without slipping (thus helping the smoothness of the drive and adding grip over the direct drive of a metal drivetrain). Also, the K1 may have been running ball diffs, not gear diffs (I found a gear diff only worked on my FF if there was tons of grip - so for a 4WD even in high grip conditions the ball diff's smoother way of providing drive may have helped here).
Then there is the motor rotation direction that may also play a role: Whereas the K1's motor can pushes the front of the chassis onto the ground on throttle, the Durango moves it from side to side. So the K1 is counteracting the weight shift of the acceleration, the Durango is not and is actually making uneven wheel loads, possibly causing the diff to activate on acceleration alone.
Despite all that, I'm surprised and happy to hear the FWD K1 did that well! The weight balance of the K1 you'd say is far from ideal for a FWD car. Could you tell me more about the track and conditions? Did it happen to be quite open and flowing or is the astroturf really that different in the UK? (I can't imagine FWD 4WDs having enough grip to work their way to the top of the 2WD rankings on astroturf tracks I've driven on in Belgium).