Also a lower roll centre has the potential to create more vertical downward grip on the tyre during cornering if the correct springs are used. If you use the same springs and you lower the roll centre the body will roll more as more load is being pressed on the springs of the same poundage. In the full size world you would go up to stiffer springs to control the chassis and tyre angles. The stiffer the spring the more load the wheel is seeing. If the tyre can handle that load you will gain grip, if you overload the tyre you will strub the tyre and the tyre will slide.
What you are dealing with in RC sometimes is grip roll on high grip tracks, therefore you probably have tyres that can grip far more that in the fullsize world and therefore you are tuning outside of the parameters of most fullsize competitive vehicles.