Your proshop guy is correct. Your arm should no be straight and locked into position. It should have a slight bend to it. Shortening your draw length 2 1/2 inches sounds like a lot to me. When you shot your bow with your arm locked... Did the string slap your arm a lot of times? It hurts when that happens and haveing a bend in your arm will help prevent that. It will also help prevent the string fromcatching on your clothes when your wearing coats or alot of clothes to stay warm.

Chief