Stuart unsprung weight is exactly what you say, weight that is not supported by the springs.
Big wheels and tires, and big brakes, can also add unsprung weight.
The more unsprung weight the less the suspension will be able to control the movement of the tires. The more unsprung weight the more each bump might bounce a tire away from the pavement, and decrease the amount of control that tire can offer.
Its probably as much engineering 101 as suspension, because most of this issue is the impact of the added weight on moment arm calculations.
The weight of a rack and pinion is not likely to be huge and neither is the impact on ride and handling. But given the limited advantages of rack and pinion steering I would not sacrifice any of the ride or handling capabilities to achive it. I'm not willing to believe the benefits would exceed the negative impact on ride and handling.
And I sure won't be paying that much money for suspension changes that degrade handling. If any of you try it I'd like to hear about how it drives once you're done.
