There might be surprises in the interpretation of the halo for next year, perhaps as an additional aero device. This will also include any new vanes/fairings on the monocoque to obviate aero losses due to it. There could even be use of the halo to refine airflow to the rear or to provide downforce (particularly with the T-wing disallowed).
On the performance disparities of the cars; it is not as simple as oil burning or rake angle. Its a whole system of things usually driven by software e.g. applying the right amount of torque to the rear wheels, algorithms determining the parameters of harvesting by the MGU-H and MGU-K, supercomputers accurately correlating CFD and dyno simulations so that they meet expectations in real life use etc. All Information Technology (IT).
If Mercedes edge is in IT, then it could be something inherited from the vast resource base of its parent, Daimler AG. Information Technology (IT) depth is not easy to overcome, so if that's Mercedes advantage, they are likely to keep it for a few more years; rake or no rake, long or short wheel base.