You are probably 'feeling more rear body roll' because the stock springs are softer and will not increase the rear roll rate like stiffer rear springs. 'Roll rate' is an expression of the stiffness the suspension exhibits to the cars tendincy to roll in a corner, and is is exacly like a spring rate; think of it beingn like a T-bar twisting stiffness.
Roll rate on both front and rear come from 2 sources: in the rear, it is the rear spring stiffness plus rear anti-sway bar stiffness. On the front, the roll rate comes from the T-bars + any front anti-sway bar. The overall car's roll rates is the combination of front plus rear roll rates. So to get it right, you have adjust the anti-sway bars to get the right roll stiffness with the springs you have on each end, and distribute that roll stiffness front to rear for oversteer-understeer.
What you are likely experiencing is that you have stiffer t-bars up front, and these are now comprising most of the car's overall roll rate. The rear is just going to 'wallow' around more as corner tightness changes and throttle and braking change due to the lighter stock spring stiffness. As the rear height moves up and down with these varying corner conditions, the rear roll center moves up and down and the over-understeer characteristic changes, and things move around even more. Stiffer rear springs combats all of this. And a rear sway bar helps take the roll load off of the front and (can) further help the rear move around less.
I have experienced this type of thing long ago on a race car when younger and poorer and using some stock progressive rate springs.
