Here is a very handy chart on sway bar sizing and percentage difference. [chart sourced from whiteline suspension]

from the chart, going from a 18mm to 25mm sway bar is a increase in stiffness of 272%




cheers
Benny

My dad is a big believer of stiffening the car and using softer springs and valving, he doesn't like the way I have set my car up at all. I say go for it, doesn't hurt to try these things, but (you have konis don't you?) I'd set the valving softer and maybe look at trying some 200lb springs.

What are your alignment settings?

this is different to what I'd been going by. an old suspension book said to compare rates of different sway bars you use the cube of their diameters. But this chart gives different results than that does. Whiteline chart added to my modification bible - thanks Benny. That actually changes what I though I needed for a front swaybar though....