Great thread Khaled.
Müller "only" scored 525 goals for Bayern if we only include first team appearances. Indeed he scored 41 goals for the B team but those should not be included in the official count. If so then all the goals scored by Messi for the Barca B and C teams should be included as well.
Messi needs to score 274 goals more to beat Müllers record for Bayern.
Let us just say that he will do exactly that in the next 5-6 seasons. To do so Messi need to score about 50 goals each season. If he stays healthy then I don't see why he should not be able to do that.