IMHO the front strut bar makes a bigger difference than the rear bar. The rear of the car is fairly stiff compared to the front, so less support is needed, not to mention the rear suspension puts different and often smaller loads on the strut towers compared to the McPherson front. If you have upgraded front springs and a front swaybar, the strut tower will make a noticeable difference. On my car the turn-in became much sharper, as opposed to the chassis having to take a "set" before tracking through the corner. Of course, the mileage and condition of the chassis will dictate how much flex there is and therefore how much effect the bar will have. Sunroof cars have slightly less rigidity (probably inconsequential, but for the sake of argument...) and also have a higher center of gravity pushing them further over in the corners (again...) and so possibly benefit more.
