If your chassis was originally made for a 150hp'ish engine, beefing up the suspension and brakes may be sufficient for below 250hp. However if you're gonna cross over to 300hp mark, chassis rigidity must be considered. Some strip the car totally naked and re-weld all the factory spot and added roll cage bal bla bla. Then you need also think about changing all the bushes and upgrading the lower arm etc. You have to do that before thinking about going to 600hp. At least this is what I would do if I want to push it to 600hp mark.