I can't give you any specific advises without knowing details about your infrastructure.
ubb.Threads doesn't support MySQL replication builtin, but it's possible. I developed that for a client some time ago. If you're interested in that, just drop me am PM or email.
2000 simultaneous users is quite much, how many posts do you have in your database? I know some boards with about 2mio posts and up to 1000 users online.
Your keybuffer is very high, you should lower it. No table in ubb.threads has such large keys. Better define join_buffer and read_round buffer instead.