I drove around in a JA for 5 months with no power steering.. its not really that bad. at first it feels a heavier (obviously), but if you shave your head you can pull off the vin deisel look when your biceps pump out to 3 times the size they should be
I've heard that a Sigma steering box will bolt in too. It's too bloody hard to steer without PS running on a staz. Swap the box and it would be a little heavier to turn but not much, it's not like there's a huge amount of weight to move around.
The Sigma box should interchange but I think the ratio is different. You would have to change the Steering Arm too as it is shorter on the Sigma ( about 20mm I think ) but more importantly the Sigma Steering Arm has the tapered hole in it, the opposite to the Starion set-up. If you had a stuffed Starion Drag Link you could probably convert everthing across from the Sigma, idler arm inc. but your sterring would be slower. Also don't know what affect it could have on your bump steer geometry.
Could you shead some light as to why you want to go to manual steer? The power steering uses about 2-3 Hp on static turn , when moving it takes about 0.5-1Hp depending on what speed your doing, surely you can't be chasing every little bit of power? :beer
The power assisted steering pretty much turns off after 80km/h so there shouldnt be a problem with predictability
I love the steering in my car, one of the best setups i have ever driven.