If the rear springs have too high a spring rate in proportion to the front then it will be more prone to oversteer. By fitting the 4 or 4.5Kg rears you may find that you will loose that balance you have now. I personally suggest fixing the actual tyre scrub issue instead of just trying to avoid it. I do find it odd that there is such a difference from having a passenger alone, well unless they really need to loose some weight.

You are running 4.5Kg in the front right now?

In the past on my AE86 I tried running 6kg front springs but the stiffness drove me crazy. I ended up choosing 4.9Kg / 275 Lb which I loved. But as you said the higher weight over the front end would have an effect on this, as for how much I have no idea.