Underated or not, its still not doing 189Mph. Even if it had 430Bhp real power, It wouldnt do it. A 5.7 03-04 model with 350Bhp had a max of 167Mph(Limiter cutoff 158Mph)(I know stupid)
A Hsv Vz GTO Coupe is said by Hsv to hit 180Mph even, Most have reported 176Mph, ANd that has the same engine, and gearing(With exception of 5th and 6th) and "Slightly" better Aero, And that doesnt do 189Mph.
2005 C6 400Bhp 187Mph, 2004 911 Turbo 415Bhp 188Mph, 360 CS 425Bhp 186Mph, 2004 493Bhp CL55 AMG Unlimited 189Mph.
Fastest stock 2006 400Bhp GTO i believe has been 172Mph, SOme "Speedos" showing higher. This does not do 189Mph plain and simple, Not with what it has.