Dropping the rear suspension on an IRS Bug is pretty much the same as a swing-axle Bug, the only problem is you can't see the torsion bar due to the shape of the spring plates.
What you must know is that there are more splines on the inner end of the torsion bar than the outer. A drop of one inner spline will give roughly a 2" drop, turning it by one outer will drop it considerably more.
Or just do what I do and fit adjustable spring plates.

