I don't know the Tamiya springs, so i'm not sure how hard they are. One thing you must remember is that you want the car to maintain a level of balance front and rear. For example, a 1.6 front sway bar, but none in the back may create excessive roll in the back, so maybe add a 1.3 rear sway bar.
Also, having the same oil front and rear, but different springs, will create a different damping effect. If you have a stiffer front spring, have thicker oil to go with it.
One other thing to do if all else fails, go back to the factory set-up. It's not bad for many tracks.
Agama A8 Evo | GRP .21 | Spektrum | Savox
My car just hit 88mph. I hope it comes back soon....