bingo... you nailed what i was thinking, actually
i did this for another large forum, where they had 3million+ posts..
just removed those two indexes and replaced search with a modification to use google search instead...
i also auto generated a sitemap for ( google, bing, yahoo ), so all topics, forums and categories stayed current and quickly indexed.
the google search also has the added benefit of allowing them to place adsense on the search results ( cha ching )
or you could just go with super slow ( internal ) search that is one of the options in control panel..
so basically, just drop those two indices in the ubbt_POSTS table...