If a GM always uses the same buff sets and opening strategies with Arcane casters, because they are effective mind you, is it metagaming to have your characters use strategies to counter those NPC strategies even when they don't know that this sorcerer/wizard/... has actually used the standard stratagems?
No. In fact, I'm pretty sure every military in the history of ever would prepare for facing commonly employed tactics.
If a normal person keeps coming across the same circumstances over and over again they will begin planning for those circumstances no matter how "unlikely" their recurrence may actually be.
The GM presents the realities of the universe to the PCs. What happens to those PCs in-character is the only grounds they have to predict the future. For players to recognize repeated tactics is just and right, not metagaming. Now, if you start expecting different tactics because the GM has had a conversation with you and have your PCs anticipate a change... that's metagaming.
OK, thanks guys.