Hmmm! What was the reason you couldn't get more than 155km/h?
Was it the road condition or the engine couldn't be revved high enough to produce enough power to go above 155km/h??!!
On normal flat highways(if you do), 155km/h is easy for a GQ within seconds after take off.
I'm guessing Wanneroo raceway hasn't got enough long straight stretches to reach high speeds?!