The best Japanese restaurants have vastly superior sushi including sushi rolls.
However, a lot of more expensive but not exactly top end restaurants don't have top tier sushi. Maybe a bit above average.
As for buffets, there are some really bad ones, and some good ones, and if it's a good one it can be as good as a non-buffet restaurant, esp. if you get the stuff right after it's made. However, usually the selection sucks, and the expensive features won't be offered, and it will never be as good as a top tier Japanese restaurant. Unfortunately, top tier Japanese restaurants are $$$$.
BTW, the uber high end Japanese restaurants don't even give you wasabi. I know most of the restaurants give it to you, but it's actually considered a bit of an insult if you automatically put wasabi on everything including nigiri sushi. You're telling them it's not flavoured correctly, even though they already have wasabi on them.
It sort of like pepper. Some uber high end restaurants often don't give you pepper or salt on the table. The food is supposed to have been seasoned correctly, so if you add pepper or salt without even tasting the food first, that means you know the chef can't season food properly.