I have an '88 951 S with the 030 (club sport option). Does anyone know what size sways came stock on the car?

Thanks.

 

26,8 mm front / 18 mm rear.
The rear was switched from 18 mm to 16 mm during the '89 Model Year.

 

So will i notice a big difference if I go to the 19mm rear? Better or worse?

 

It will be "better".
As you may already know the 19 mm is adjustable so you can set it to how you like it best.
I would first make sure the rear shocks are in good condition and do not need replacing. Fitting a bigger sway bar with shocks in average condition is not the best thing to do IMHO.

 

Best way to test shocks? I do notice that the rear seems to squat more on accelaration than my 944S did although I also know there is a lot more power in this car so it may be normal.

I had the bar omn my 944s on the mediem setting. I did like the way that car felt.