this seems really good at one mana

You reckon? Sorcery-speed Battlegrowth, plus "target creature gains haste" which is unplayable in every set?

Counters are important in this set, the 1 mana counters are very powerful--I remember playing this at 2 mana to get critical mass for Barstool Ruffian (then a 1 mana 0/2 I think, or 2 mana 0/3)

It's definitely playable, but it's not "really good".

Would being an instant be that bad?

you know you are on a supply as sorcery bent at the moment but the speed kinda kills it imo.

Well there's a reason I made these sorceries. They were too good as instants in playtesting. Also, the haste part has no reason to be instant speed, so making it an instant is kind of awkward.

Its just so minor an effect without being able to pull it as a combat trick. Maybe a late pick when drafting red but i wouldn't really call it constructed playable

I meant playable in limited. +1/+1 counters matter more in this set than most. It's not a great card in limited, but if you have a +1/+1 counter matters deck going, you'll be happy to pick it up late.

Instant speed seems like a bad idea to me. People will hold it in their hands for a trick and the card would rarely do what it was intended to do... give a creature Haste.