Not under powered. The new wing will fix most of the 300ER's woes.
The basic airframe of the 777-9 is lighter because of the carbon wing. Lighter airframe, better payload/fuel capacity with heavier loads and better range.
The more efficient wing will carry the same weight, with less power needed.
If we cut a deal with Boeing on the Max debacle, I'd much rather see an order for the new 777's as opposed to more 300ER's.
Now, if they would just get us a radar that actually works in auto….instead of being called scary mode.