Underperformers don't get into raids and are given the option to remain in the guild as a non-raider until they get the time and/or gumption to put in the effort to earn a spot. This is made clear from the day they tryout with the guild, and is explicitly spelled out in our guild policies.
You "expect all raiders to be raid ready" - are these expectations written down in a policy? If so, /gkick. If not, you have a problem that needs to be solved by your officer corps. One of the most important positions in a raiding guild is the person who writes down and maintains policy documentation. Cue the people crying wtf, it's just a game!