Sentience is one of the basic goods. If the sysop is non-sentient, then whatever computronium is used in the sysop is, WRT sentience, wasted.

Not necessarily. It depends on what the sysop does with all that computing power once it's in charge. Sentience is one of the basic goods, but having whatever sentient creatures exist live excellent lives is another one. If the sysop uses 60% of the computing power to run itself and 40% to run sentient creatures, it could still be a net win if the sysop spends most of its time finding new ways to make those other creature's lives as wonderful as possible.

Look at it another way, the organic matter currently being used to make your clothes, food, home, etc could probably also be used to make more humans out of. But it's probably better to use it to improve your life then to create a bunch of cold, naked, hungry people.