How does a Circle of the Shepherd’s Mighty Summoner feature interact with the UA spell Summon Bestial Spirit?

Our DM has allowed us to play-test the summoning spells which appear in Unearthed Arcana 2020: Spells and Magic Tattoos and I have added the 2nd-level Druid spell Summon Bestial Spirit (pp. 3-4) to my spell list.

I chose the Circle of the Shepherd which has a feature at L6 called Mighty Summoner, which is meant to enhance a druid’s summoning/conjuring spells by adding +2 HPs per Hit Die and causing the damage to be classed as magical (XGE, p.240).

For example, with the 3rd-level spell Conjure Animals (PHB, p.225), if I conjured a Giant Boar (MM, p.323), it would have an additional +10 HPs because it has 5 Hit Dice (5d10 + 15). So the conjured/summoned creature would have 5d10 + 15 +10 HPs in total.

Here are the relevant parts from the UA spell:

Summon Bestial Spirit

You call forth the spirit of a beast. […]

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the creature assumes the higher level for that casting wherever it uses the spell’s level in its stat block. […]

Hit Points equal the beast’s Constitution modifier + your spellcasting ability modifier + ten times the spell’s level

In the case of the Summon Bestial Spirit the creature’s HPs are linked to the spell level, so how does it interact with druid’s Mighty Summoner feature? Does the spell level determine the Hit Dice?

I would appreciate an answer with an example and/or experience of having play-tested this spell.