To take advantage of all of current tech a 150hp V6 would be pretty tiny. less than 2L.
But it would still have more mechanical losses due to two sets of cams, 24 valves, and the associated losses of the two extra cylinders thumping around.
It would be an interesting exercise.
The Honda Goldwing uses a 1.8L flat 6 with about 100hp and 108ft-lb of torque at the wheel. It's not known for great fuel mileage, but bikes are less aerodynamic than cars in general.