The +5C temp cut would certainly help but I feel that you havn't got true control of the flow temperature, its well named as target temperature, vaillant state that its the return temperature (probably the deltaT) that modulates the boiler, IMO Flow temperature control allows the flow temperature to increase very rapidly and as it approaches the SP temp it reduces the firing rate (derivative control) to avoid any or very little overshoot, this is what I see happening on my relations 20 kw Vokera VisionS where even with a 5kw demand and no range rating the flow temp only overshoots by ~ 2C. and the settles down but it has that +5C margin.