@Vadim, you are going to have a rough time with those 280lb springs for autox. Especially if it's a fast transitions type course.......And although Kojima is spot on, I do not recall him recommending a spring rate. So, if the fronts are 20-30% of 600lbs, you are looking at 420-480 lb springs. I would personally stick with the 400lbs and tune in rotation using something else (tire pressures, track width, etc)
Pucshoe was using 650lb spring all the way around........And typically, with the low speed stuff, you need fairly fast reacting setup.
I could be wrong and you could be crazy fast at the next event. But if not, that is probably your reason
P.S Looks like you were under-steering in the above pic