The Z428i is listed as as almost 300lbs lighter than a Z435is (3263lbs vs 3549lbs). The rule of thumb is that each 100lbs is worth roughly about 1pt. So that is 3 points. The Z435is has more torque (we mostly use torque, not HP) and that accounts for why it is only 2 points and not 3 points. Keep in mind that the benefit of HP/TQ (and points assigned for extra HP) drops off as we get into higher HP/TQ. The 28i engine already puts out a pretty healthy 260ft-lbs.