The ideal launch rear height would have the halfshafts in a horizontal position inline with the axle spindles and diff. If the halfshafts are at an angle at a hard launch, they or the joints will be more susceptible to breakage. It's a dynamic situation including traction and ride height. It's typical for IRS to squat for the sake of halfshaft geometry. I think you need to review a pic of your car at launch to determine how much it squats and establish the halfshaft angles at that height, from there you can adjust the dampers more or less compression to straighten the shafts or if needed.. replace the springs if the damper adjustments cannot accomodate the target launch height. If you already know this.. no worries for wasting my time babbling..
mj is right about all around driving, traction is different on a prepped strip compared to a dusty street..