I'm a bit confused about the turbo+OC

Hardwarecanucks and techpowerup both write that the OC offset leads to increased turbo headroom and activity and that the (guaranteed?) base clock cannot be increased. This would mean that depending on the circumstances you won't necessarily get the most from a given OC (i.e. 15% OC equals 15% performance increase).

However, the advertising slide for this feature shifts the whole curve further up, including the working point "base clock" and the subsequent turbo frequencies. That seems to imply that the new base clock=old base clock +offset is guaranteed. In some reviews OC gives proportionable performance benefits even in very demanding games, in some it does not.

Now what is the deal with that?