What would the difference in damage taken be if you were to exchange defense EVs for HP?
Heck, you could even go so far to lower the SpA EVs to the point you can reliably KO Breloom, and run more HP while keeping the defensive EVs.
If you exchanged Defense for HP, attacks you might commonly encounter like Machamp's Payback and Scarf Tyranitar's Crunch will allow them to KO Exeggutor after they break a Substitute. Additionally, Leech Seed and Giga Drain will heal a smaller portion of your overall HP.
For specific calculations:
Exeggutor as I have built it:
Adamant Breloom Focus Punch: 36-42% (cannot 2HKO with SR)
Scarf Tyranitar Crunch: 71-85% (cannot KO after SR)
Adamant Machamp Payback: 64-76% (rarely OHKOs after breaking Sub)
Exeggutor with HP instead of Def:
Adamant Breloom Focus Punch: 39-46% (can potentially 2HKO with SR)
Jolly Scarf Tyranitar Crunch: 78-92% (can potentially OHKO with SR)
Adamant Machamp Payback: 70-82% (often OHKOs after breaking Sub)
Remember the object of the set is not generic survivability (where HP is usually maxed), but to specifically counter a certain Pokemon that ordinarily never goes mixed. Furthermore many of the common switchins to Exeggutor also use exclusively physical attacks (usually Pursuit or Payback), or else the Pokemon Exeggutor threatens outside of Breloom use them (Machamp being the primary example because you also cover its Dynamicpunch).
I guess I should note that the 244 HP variant does survive Technician Life Orb Hidden Power Bug, but if Breloom is resorting to that to deal with Exeggutor, mission accomplished lol.
As far as the special attack is concerned, Breloom could probably be defeated without any investment, but the point of the set, while it does counter Breloom, is also to take advantage of Exeggutor's marvelous 125 Base SA and threaten the opponent from behind a Substitute. You need all that SA to get the OHKOs/2HKOs on Machamp/Tyranitar.