Using a Stat Calculator with all five races as a Summoner with no subjob, the increases per boosts were determined. All MP Boost traits add an additional 10 MP.
Confirmed that Max MP boost does stack with MP merits at the merit level. For example, say you have 8/8 merits on your BLM. If you start leveling SMN (which gets this trait at level 10) and your first max MP merits also become available at level 10 you will receive an MP boost of 20 + the standard level increase.