What do you mean by "pretty good"? Ichiriki recently won a game against Lee Sedol by 0.5, and that's about the only notable win I could find between Ichiriki and Yu against international competition from Korea and China. Looking through their records going two years back, they both lost consistently against middling competitors from China and Korea.
Well, perhaps I pay more attention on their wins whereas you pay more attention on their loss. I don't say that they are already on par with, or better than Li Quincheng or Shin Jiseo (like I never said that Iyama is stronger than Ke Jie or Park Junghwan, or Lee Sedol), but I would expect that if Ichikiri and Yu (and even the others like Kyo Kagen, Shibano or Hirata) can play more international tournaments in the near future, their level will be not far from their Chinese/Korean counterpart. If you want a list of wins:
- Ichikiri won 3 games in the Nongshim Cup last year (1 in the year before), got a 6-1 record in the Chinese C League. IF you still argue that those opponents are only "weak Chinese/Korean pros", he just went 1-1 with Yang Dingxin in 2016 (a top 20 Chinese, 34th on goratings), and kicked out Li Quincheng in the Samsung Cup few weeks before his game with Lee Sedol.
- Yu lost a bad game to Lee Sedol few months ago, but he won to both Chinese and Korean participants in the Asian New Star match recently (Dong Mengcheng is a strong guy who just kicked out both Shi Yue and Gu Li just before the match). Yu also beat Peng Liyao and Lee Donghun (no.2 Korean youngsters) in the LG Cup last year, before failing to the runner-up Park Younghun.
About the chance of a Japanese win a Samsung or LG Cup, well, as their young talents are now at best world top 50-100 level, I would say that the only guy who has a slim chance is still Iyama, but perhaps he needs to play like 10-20 tournaments to really have a chance to win one (so do most of the strong Korean/Chinese players outside top 5).