I have just tried that car with all assists off, I use a csr wheel and pedals.
It is no more tail happy than other cars with that sort of power, I have to be very precise, even tentative with my throttle usage to avoid major spins and drift racing. I suppose all I can say is and don't say I am right as I am not the best forza driver with no assists is I have found to find your correct braking points, almost free wheel your speed round the corners with a touch of throttle if you start to drop off, wait til car is steady and on exit of corner increase throttle, hard to do on controller, but I find any car with no assists when the throttle is applied when cornering never ends in a fast lap. Good fun but not fast