this should not be the case at all. It is unsprung.
IF anything, the disc will weigh less and allow the car to go faster.
IF it was "Sprung weight" on the car, then you have something.
Dr.Diff has kits that will help out. You can re-use your old wheels and tires. Not an issue. Upgraded axles can also be had at Dr.Diff but IIRC, only in the larger 5X4-1/2 pattern. Upgarding the front discs to a 5X4-1/2 is also an EZ kit away. Factory or not.