I have flown 737 and A320. Not 777 unfortunately, but in terms of discussion I think its better to leave the fancy computer bits out of it anyway.
But as you may remember from small aircraft during training, if you trim the aircraft for a speed in level flight and you add power, eventually you will end up climbing as the pitch attitude increases to maintain speed. This is obviously slower and likely less pronounced then the pitch up effect from underslung engines.
When someone is 15 kts slow I call go-around. No point trying to save that mess.
But for the sake of the discussion I'll oblige.
Offcourse you need to add power and a lot of it as well since your on the back side of the powercurve. As the speed increases the pitch attitude reduces in order to maintain the path.
But what if you are so slow that you at a stall, you will lower the pitch attitude to reduce the aoa and thus increase airspeed. Only when that is under control you will slowly add power. Thats the new stall procedure according to the same manufacturers.
This example however has very little to do with a normal stabilized approach in windy or thermal conditions. And none of the manufacturers would advise us to get in a Vapp-15 situation.
The actual performance of the aircraft= pitch + power + external conditions (up/downdraft, gusts, tail/headwind etc)
The required performance is usually a 3 degree glidepath at Vapp.
As the external conditions vary you can only adjust pitch and power to match the actual performance of the aircraft to the required performance. As Vapp is fixed, pitch and power are linked. And it doesn't really matter which one is changed first.