30mm - OE
35mm - GP - 17% larger
26mm - OE
33.4mm - GP - >28% larger
The OE (production) bars were used on a number of vehicles with different tire/wheel combinations, springs, and shock calibrations--any 91-96 Chevy sedan with F/R bars as part of the suspension RPO
used the same bar set. This bar combination is fairly tame, and tends to provide a relatively "understeering" combination, with the rear bar helping to reduce body roll.
Compared to the OE bars (and assuming the OE springs are still being used F/R), the GP sedan bar package will move the handling balance more toward neutral to slight oversteer, depending on the wheel/tire used--Impala SS wheel/tire might exhibit LESS oversteer at the limit than a smaller wheel/tire, since the greater roll resistance with the larger rear bar could more easily overcome a smaller, narrower tire's traction limits.
The larger bars also reduce total body roll more than the OE bars, which helps keep the front tires planted with less positive camber gain, thus the tires work better. There are other variables, some which could negate what I just stated about the Impala SS wheel/tire, and it could be a long discussion to sort through them all--tire pressure, alignment, F/R weight, fuel load, road surface, traction limiters (moisture, oil, etc) and so on--to name a few.
Bars do come with all installation hardware--rear GP bar CAN work on stock LCA with OE 2nd design mounting hardware
. The mounting hardware included with the GP rear bar is mainly for use with aftermarket rear LCA's.