Bigtime strain on the engine, and you'd be in a non optimal gear so where you are in the powerband would not be pretty, in other words you'd be in a highly inneficient area of the RPM band to be going WOT. This means poor MPG also.
Like said above, RPM is friction to a point as well, peak fuel comsumption versus RPM occurs guess when, peak torque output. Increase the RPM and you increase fuel consumption, but likewise running too low RPM is not only risky for proper oiling, but you will put the engine out of its efficiency range if your trying to cruise at 500 RPM with tons of throttle.