basically rollcage purpose is to protect the driver from being crashed / thrown out from the car in the event of an accident. fundamentally when rollcage points were bolted on to the chassis, the rigidity level will increase when all the stress / body roll during cornering transferred to the bars. so yes, indeed it will improve the cornering capability of a car.
the burning question that has been left unanswered for a long time is "is it legal to bolt on rollcage on our daily driven, street car?". If yes, which regulation shall be followed and what specification is approved by the authorities. If no, why cars with OEM-fitted rollcage are allowed to be driven on the road?
just my 2 cents...