The question is how much efficiency is lost to "oversizing" the engine (compared to steady flat cruising needs) and how much would be lost to a hybrid system.
Looking at this graph, if we assume the car needs a steady 15kW (about 20hp) of power to cruise at a steady highway speed, it would use at best 260g/kWh (about 32.4% efficient) with perfect gearing. But the most efficient the engine would be would be 245g/kWh (about 34.4% efficient) and would give at most 58kW (about 78hp) of power at that efficiency (with perfect gearing). The most power the engine could produce is about 85kW (114hp) but would use around 300g/kWh (28.1%) with perfect gearing.
But if you were to extrapolate the numbers to accomodate a +200hp engine then it could easily fall to the 300g/kWh ring just cruising.
