Simply put, the longer the arm, the softer the effective spring rate. Similarly, the more offset, more leverage on the axle, softer rate. Most torsion bar manufacturers can give you a conversion chart that calculates the rate change for a given bar diameter/arm length ratio.
As far as I know the arms don't go bad, but torsion bars will; if you live in the Indy area Competition Suspensions (CSI) in Brownsburg can dyno your bars and tell you how tired they are.