I have no clue on what pressure the Cosworth pump should do. But 100 bar doesn't seem low to me. I know that my BMW 130i has a pump that does 120 bar and that car is a bit heavier than a Sierra and runs a lot more castor, so that makes the steering heavier. But apparently running with 120 bar is no problem in that case.

I guess your problem might be the steering rack and not the pump. It seems that quite some people have heavy steering on 2wd Cosworths, so I'm guessing the torsion bar in the steering rack is just stiffer than the one on the 4x4, or the one on a normal 2wd Sierra rack. I now use a normal 2wd Sierra rack with custom made lock stops and it steers a lot easier than it did with the Cosworth rack. But I do believe there might have been something wrong with my Cosworth rack.