I'm not sure how flexibility is going to help your Elbow strikes. When striking with your elbow, your shoulder joint never moves too far in any one direction. If you can touch your left shoulder with your right hand, you have just moved your shoulder joint well past the point it needs to go for delivering an effective elbow strike.
Elbow strikes are about timing, you can't muscle a good elbow strike. Your body has to move in good form and create the most thrust just before impact. .
Maybe I'm not understading what you mean by "flexibility".
-BRR