The Racing Beat anti-roll bars are not adjustable, but are considerably stiffer than the factory parts. The rear bar, for example, is approximately three times stiffer than stock–we calculated a bar rate of 900 lbs./in. vs the stock 300 and change. Note: the effect at the wheels is much less than this, as the bar attaches to the suspension arms about mid way down their length, making the wheel rate approximately half that amount.
The Racing Beat front bar is 28.5mm, made out of solid spring steel, whereas the factory piece is a 22mm hollow bar. Bar calculations are a little funky with all of those bends, but the rough estimate is 780lbs/inch for the RB and 350 for the stocker.
We use http://www.auto-ware.com/calcs/swaybar.htm to calculate rates of anti-roll bars.
The front bar's end links are attached to the strut housing, meaning that the bar rate is much closer to the effective wheel rate for the Mazda3.