I own a city car Chevrolet Spark 2016. When going up a steep hill say in 4th gear, having the throttle down at 60% or WOT is practically the same. There's a minimal increase in RPM and power. Will the vehicle consume the same amount of gas if at 60% rather than WOT? Is going WOT in this case harmful for the engine? Is it better to shift to 3rd and slowly gain speed or to 2nd and go faster, quicker?
I would bet the engine in your car is drive-by-wire, which means the computer is controlling the throttle, not you. In doing so, it is going to look at the load put on the engine and a bunch of other factors and give it only the throttle it can use as well as the gas ... at least in high gear. By going WOT in this instance, you'll not be causing your engine any damage beyond the normal wear/tare.
As far as mileage goes, you will get worse gas mileage by downshifting than you will by keeping it in the high gear. This is because the engine will be revving to a higher level. More RPMs = more gas used. The difference is, you'll most likely be able to obtain a higher speed with the vehicle because you'll have more available hp/torque to the ground. It's the age old trade off of get there faster or get better fuel mileage ... take your pick.