Somewhere, I don't remember anymore, I was advised to use 250 #/in springs in the front and 130 #/in in the rear. When I drove it, it seemed to be darty when there was uneven or undulating pavement. I changed to 150 in the front and 115 in the rear. That's not a whole lot stiffer than stock but as I say, I have 15" wheels and that adds to the effective spring rate. That seemed to settle it down a bit.
Now, you have to understand that I make NO claim that I have any skill in setting up suspensions. It also could be that I may have had some alignment issues that I wasn't aware of. And then, I could have just freaked out a little bit because it didn't react the way I remembered (25 years ago!) but I certainly didn't have the same car!.
250 #/in in the front seemed high when I heard it but since then, I've heard of others who run that and seem happy with it. I would expect that they are running 13" wheels - like you. I don't track or autocross my car so I don't need the last tenth on handling.
I would tend to trust whatever Richard suggests and I haven't heard anything negative about those tires so I think you should first satisfy yourself that your alignment is correct and there is no binding or slop in the suspension. Also if you did anything with the steering rack, you may want to revisit that. I rebuilt my rack and didn't adjust the tie rods tight enough and it would want to change lanes on the highway with the slightest steering input.
Hope that helps!
