Rough ride/steering control/Suspension adjustment?

[insatiable]

During the 1900km. return trip after purchasing my 03 Box. ( has 18" wheels,Cont. Tyres, and 15000kms. on clock) I had to drive on some roughish "black top" outback roads. The car ride was rough in the extreme but worse the steering at 100 to 140kph. was being deflected by the road surface due I suspect to the suspension not yielding enough. It was very uncomfortable and at times bordering on dangerous.
I checked tyre press. and they were cold, F.29 & rear 36. psi.
There will be many times on these sort of roads so I was wondering if it is possible to reset the suspension or would going to 16" make an appreciable difference. This option would have to yield a real benefit as I would have to buy a full 16" set.
Incidently I have driven these roads in other cars and although rough they have handled it well but I do understand the Box will have a much firmer ride.
Cheers all!