not true, there are several peeps at svtperformance.com that are running 10s on the stock rear. does it slow them down? a bit, but theres no question that as long as 11s are as fast as you want to go its better than a live axle.
429... go over to the aforementioned site and look at what the Kenne Bell and Procharger guys are putting down. Many over 600 at the wheels, with only the basic boltons to the rest of the car. 650 is probably the topend of that, but these people are also driving their car to and forth from work every day.
i'll try and find the link for my claims, brb. remember that with a cai, pulley, and catback, these guys routinely pull 450 to the wheels, and that KBs and Prochargers can net huge gains on top of that.