I think you'll be fine. Their title 'Masters' swim is a bit misleading as the session is open to all just as long as you can swim 400M continuous freestyle \ front crawl.
I used to go to their Tuesday Masters session and I swim 400m in around 10/11mins. I was usually the slowest in the slow lane and sometimes skipped a couple of lengths at the end of a set but it was a good session. With 400m in 8.30 you should be fine.