Is there any benefit to leveling a spirit before dismissing?

Yes, there is a benefit to leveling a spirit before dismissing them. Higher level spirits yield more SP when dismissed.

SP for level 1 primary spirits

1. Novice
   100 SP
2. Advanced
   200 SP
3. Ace
   400 SP
4. Legendary
   800 SP

SP for level 99 primary spirits

1. Novice
     500 SP
2. Advanced
   1000 SP
3. Ace
   2000 SP
4. Legendary
   4000 SP

The above list is valid for all primary spirits, regardless of their Type (Red, Blue, Green), Power, Skill, Support Slots, or whether they can be enhanced or are already enhanced.

Since enhanced spirits normally have a higher star rating than their base version, enhancing them may increase the amount of SP gained when dismissing them, assuming you intend to level up the enhanced version, that is.

Keep in mind that feeding snacks to spirits costs SP. It costs 8 SP per 250 experience points, and the amount of experience needed to level up increases with their level and rating. Using Snacks to increase a spirit's level before dismissing it may therefore diminish your returns, or even incur losses.

Not part of the question, but for completeness:

SP for support spirits:

1. Novice
   1. 100 SP for 1 slot
   2. 100 SP for 2 slots (this is not a mistake)
2. Advanced
   1. 200 SP for 1 slot
   2. 250 SP for 2 slots
3. Ace
   1. 400 SP for 1 slot
   2. 500 SP for 2 slots
   3. 600 SP for 3 slots
4. Legendary
   1.   800 SP for 1 slot
   2. 1000 SP for 2 slots
   3. 1200 SP for 3 slots

_{Since I couldn't find any 3-slotted Novice or Advanced support spirits, this list may be incomplete}