As far as Japanese food goes in Sri Lanka, I think Nihonbashi has it in their corner. Food has been constantly great and the service is brilliant (they really make effort to take care of you). I would say it is quite pricey but if you catch on fast you can identify a couple of good items which you can order and not break your budget.
1. Try to call ahead and reserve a room, it'll enhance the authentic Japanese experience. It's also great if you have kids.
2. When going for lunch order a set menu, it'll be worth it. Get them to bring your sencha tea during your meal because you will be too filled to have it afterwards.
3. If trying to budget stick to sushi rolls and onigiri. There is a good collection, it's filling and won't break your budget.
4. If your trying to minimize expenses, don't order drinks, stick to water.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.