There are a myriad of threads on barrel break-in, and several schools of thought on it. Here is a recent one if you are interested: Copper Removal Technique: Break-in vs Complete Removal
As far as velocity, there is another even more critical factor than velocity or mass: accurate delivery of said velocity and mass. Most of the posts I have seen "playing down" velocity usually have some context. For example: I would give up 100fps in exchange for improved accuracy, in exchange for longer barrel life, in exchange for a short-action platform, in exchange for not blowing my face off with "hot" over-book reloads, etc.... In a vacuum, faster is almost always better, in reality it needs to be weighed against other factors.
It is also worth noting that velocity loss of a particular projectile is a key consideration for many of the truly long-range guys that haunt these forums - both for trajectory and energy-delivery reasons.
Hope that is useful in context of your post?