See Gruul Nodorog.

• was too expensive compared to Falkenrath Celebrants, Shatterskull Recruit, Granitic Titan, Ripscale Raptor; green gets 4/4 with upside at ...

Dunno if this neither here or there, but would adding a "Activate only as a sorcery" at the end there be preferable? Activating it on your opponents' turns certainly doesn't make much sense.

Namely that being able to activate this at instant-speed might give the impression that doing that once the creature has already declared blockers assigned to it would do something.

Sorcery-speed might also lessen potential on-board complexity concerns (which could be notable for a common) if with instant-speed you could have some extra evaluations of mana usage with this ability along combat tricks or so on. Albeit those seem like they would require very specific conditions.

It's an interesting question with this kind of ability whether the guidance such an additional clause is helpful enough to warrant the cost of extra text.

I don't worry too much about the complexity it adds, but also don't mind the minimal usefulness of activating the ability in response to e. g. a surprise blocker with flash.

I feel I'd rather save some space in the text box.