Wrong.

Speed without weight is nothing. The only way to get more speed out of a particular bow, provided everything stays the same on that bow,is to use a lighter weight arrow and this equals LESS momentum and less penetration than the heavier but slower arrow.

The only 2 reasons to use one over the other is to get a properly spined and ballanced shaft. A shaft with a 400 spine may be just right with a 100 grain broadheadbut be too weak with a 125 grain broadhead and not give proper arrow flight in which case you would want to step up to a stiffer shaft...... say a 340 shaft.

A shaft with an overall heavier physical weight will require a heavier weight broadhead in order to give that shaft a proper FOC or ballance point.

Also, just because a particular shaft is heavier in physical weight does not necessarily mean that it is stiffer than a lighter weight shaft.