The short answer is the trans am vehicles used higher wheel rates than stock Barracudas. This was acheived with bigger sway bars, t bars, leaf springs, and the addition of a rear sway bar. However, in tune with the era, these increases in rate were no where near modern standards.
With 550 lb ft of torque, there are no suspensions out there that will be able to plant that much power with street tires if your doing a 4500 rpm, dump the clutch style launch. In that regard, the XV L2 is no better than the XV L1, or the Hotchkis, or Cal-Traks, or SS springs. However, if you learn how to finese the launch, then just about anything will handle it.
So in all honesty, I don't think you can do much about improving street launches with regular tires. If your willing to go to a d.o.t. drag tire, you will improve your lancuh ability, but will do so at the expense of lateral capability. If you want maximum traction at the strip, it would definetly be worth investing is a set of slicks, even skinny ones, one some basics steel wheels.
