Tires are 90% of the setup of any car, and i would be extremely surprised if Tamiya kit tires are the hot ticket, try getting hold of some soft slick tires such as Sorex 20 or 24, or Takeoff22. Asuming the surface is smooth and flat. If treaded tires are the hot ticket get hold of some Pit Shimizu treaded tires in 25 compound, these are very good on a smooth slick surface.

Going harder in the front could make matters worse due to the better initial steering afforded by stiff springs. try soft springs all round, and more rear toe. lower the front link, and or raise the rear link.

Give some more info about the surface and maybe we could assist further.