Thanks for the advice
Narrow wheels and tires would be preferable on the rear, even if were only for less mass on the rear. I see why lower grip rear tires might work, so it's definetely something to test! BRCA regulations don't allow it right now to use narrow wheels, hence why it runs wide rear tires.
I'm sure though that there are things to try out to reduce grip: different compounds, other profile tires, the use of different or no inserts, and of course these options should be tested against using narrow rear tires. Not to mention that a similar test should be done on the front. I guess that will happen somewhere in it's development - I can't exactly say when, but it's good to test the car on the different rubber people might run the car with.