considering that hidden posts are also still part of a thread's page contents (i.e. the 20 posts per page), hiding every post will make for a serious mess when you do get things moving. Imagine having ten hidden posts. that would mean that every time a page goes beyond ten posts, you'd pop up to an empty page on clicking the last post link. That would mean having to go back to the previous page all those times. If the RP takes off, that'll be fairly often.
Its far better to copy the stuff you want to keep into a new thread as Lillian suggested.