Alright guys, thanks for all the feedback, helps a lot!!!! I talked to him yesterday and we found out his dot uptime could be improved, too, as well as his mindflay uptime. Furthermore I told him about the specc changing and he agreed to trying it out.
Unfortunately I know where the problems are in our raid, but I cannot switch out any players (I am not an officer), instead I want to try to make them improve their performances
As a dps class my first concern is on this topic of course. I know that one of our tanks underperforms and I also dealt with some problems our Hpaladin had (now he pulls very competetive healing). In general we dont wipe on enrage timers, we wipe on boss mechanics and bad movement of some players (we are a mix of casual and hardcore players). About the general performance thing I cannot do much else than hoping they improve with practice and time.