Forretress has a base 60 Sp. Atk; that's less than Blissey's, and she can't 2HKO a Garchomp with Ice Beam. I don't think it's realistic to invest EVs in Sp. Atk, because to be able to 2HKO something, it would need at least max EV investment. Also, there's no Stealth Rock in this set; I assume you're using another Pokemon to set it up? I think it's a waste of a slot if you have 2 entry hazard setters in a team, unless the other is, e.g., Donphan, who can double as a sweeper.
The main opponents you'd use your HP Ice against in OU are:
All three have a mediocre to nearly acceptable special defence, and Thundurus will KO Forretress due the former's high Sp. Atk and the latter's abysmal Sp. Def.
Dragonite regularly carries Fire Blast/ Flamethrower, and indeed usually has Roost/ Multiscale, so it's not as if HP Ice will do that much damage to it even on a Stealth Rock switch in. It will also outspeed Forretress, meaning while Sturdy has some merit in getting an HP Ice attack, that one attack would be it.
So realistically, you only want Forretress to wall Garchomp, since it will always be a physical attacker. However, Garchomp usually carries Fire Fang, and even if you switch in on an Outrage move, Garchomp will be able to 2HKO Forretress, even without Life Orb.
Conclusion: Forretress has better things to do than carry HP Ice. You want to keep your Rapid Spinner for as long as possible, at least until the opponent's spiker has fainted. There's no point in leaving Forretress out there with its low speed and sp. def in order to attempt a 2HKO with HP Ice.
It would be a waste of Forretress' utility to invest EVs in special attack. I suggest you replace HP Ice with Stealth Rock/ Toxic Spikes, and instead find another Pokemon with higher offensive capabilities, like Ice Beam on Porygon2, Starmie, or Kyurem.