[Aussie Mike]

Sounds like you need stiffer springs. The threaded adjustment on you coil overs will only fix the bottoming out problem at the expense of your ride height. If you have adjustable compression damping you could wind this up to try and compensate. But most shocks only have external rebound adjustment or a combined compression/rebound adjustment.

Coil over springs are reletavely cheap, usually under $200 per pair so try some heavier springs. I'm not sure what weight spring you should use and it will depend on the weight of you car and the front rear bias. Someone here on the forum should be able to give you a good starting point.

Cheers