Commenting official mpg data, we must say that the MK4 always beats the MK3, low, medium, high or extra-high. I want to see further detailed tests, but I think that they will confirm the wltp pattern, with a MK4 better than my 1.3 Mk3.
Surely in highway mode the 1.5 won't stay in the best efficient range, but this is true for every engine. But while at low speed the ICE can be switched off and stay off for most of the time, so saving fuel, at higher speeds this cannot happen and this will reduce the difference between it and a standard engine. Anyway even at high speed the 1.5 98hp should be more efficient than a 1.3 102hp, since its extra-high values are better.
None of the reviews I've read so far have indicated the cruising speed where direct drive (the most efficient mode) cuts in. Will it cruise in direct drive at 1000 rpm and 30 mph (or even lower?) with the battery and motor available to provide instand extra power and torque if needed?
Reading infos from some american and japanese forums about the I-MMD of Accord and Insight, there isn't a precise speed when the system engages the clutch. This mainly happens depending from how much the gas pedal is pressed, better, from the difference between the present speed and the desired speed, because the clutch engages when this difference is low. The electric motor can help the ICE in direct drive mode, but only if the battery is well charged and only for low speed increases: if you are at 75mph, stable and in direct drive mode, and you want to increase slighty the speed, then the car will stay in direct drive and the electric motor will help the ICE to reach the higher requested speed. But if you floor the pedal, then the clutch will be disengaged and the car will be back in hybrid mode to accelerate faster.