posted by heather .
Dinner is good.
Dinner was better yesterday.
This is the best dinner yet.
I am having a bad day.
My day cannot get any worse.
This is the worst day ever.
The target is far away.
The target should be moved further back.
Whoever hits the furthest target, wins.
Is this the correct usage for irregular comparatives and superlatives?
Yes, all are fine.
I would use "farther" and "farthest" for things that involve physical distance, and "further" and "furthest" for abstract ideas. But what you wrote is also correct.
I'd also take that comma out of the last sentence. It has no place or function there.