If you've ever driven an old race car without power steering theres VERY little difference at high speed (for slight cornering, not hairpins or anything). Basically powersteering is better because it allows for a high gear ratio (eg 2 1/2 turns from lock to lock, compared to non power steering which might be 5 full turns) This helps in car parks and tight turns.
aa23 is right though, as the engine is running faster it will be able to produce more pressure in the power steering, however its very rarely that you would use 100% of the power while turning, so it really makes no difference.